Perfect focus each time using a Bahtinov Mask

[Dennis]

Hello,

I have been following a post on Cloudy Nights (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2544487/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1)about a revolutionary focusing mask designed by a Russian amateur astronomer, Pavel Bahtinov. The mask consists of a pattern of solid bars with equal width spaces between the bars. One side of the mask has horizontal bars, the other side has bars tilted at angles of ±20°. I decided to make one and give it a test drive.

Using Pavel’s formula: Focal Length/(Range 150-200) = Bar Thickness + Inter-Bar Gap, I came up with the following design for my Tak Mewlon 180mm F12 with a focal length of 2160mm.

2160/180=12mm, giving a Bar Thickness of 6mm and an Inter-Bar Gap of 6mm.

NOTE: The divisor range of 150-200 allows for a Bar Thickness/Bar Gap dimension that doesn’t come out as an awkward value of, say 6.5735mm.

I drew up the mask in Corel Draw (phew!) and printed it off on a sheet of overhead transparency film as a prototype for testing. I then designed and built a foam collar which I glued to the transparency mask and finally, trimmed the rectangular transparency.

Last night I was able to conduct some testing with the following results.

The Bahtinov Mask does work.:thumbsup:
It works very well indeed!
As you approach focus, you can easily see a horizontal “moving diffraction bar” approaching the shallow “X” of the two “fixed diffraction bars”.
At focus, the “moving diffraction bar” sits nicely centred between the shallow “X” of the two “fixed diffraction bars”.
When past focus, the horizontal “moving diffraction bar” begins to leave the shallow “X” of the two “fixed diffraction bars”.
Transparency film is not suited to making the mask as it does change the focus due to the thickness of the transparency film; visually, this was easy to see. Using Fomalhaut (1.2m), focusing without the mask was relatively precise by watching the 6 diffraction spikes produced by the Mewlon 3 vane spider snap into focus. When I fitted the mask, I had to adjust the focus by 3 or 4 short stabs on the Moonlite motor focus button – the difference definitely noticeable visually. Also, the transparency film has a sort of ground glass texture which dims the incident light and makes things a little smudgy.

So, it looks like I’ll have to make a mask with clear gaps! Many thanks to Pavel for sharing his design for this elegant, incredibly useful astro-accessory.:thumbsup:
Cheers

Dennis

[upgrader]

Thanks for that Dennis, this design is doing the rounds on the UK boards at the moment its a brilliant bit of kit. I don't agree tho with whats happening with this company http://www.spike-a.com/
The guy is ripping Pavel off plus NOT even mentioning him as the inventor, this is shameful beyond belief and will only put off people in the future from sharing ideas.

[acropolite]

That's very interesting Dennis I might have a crack at making one. :thumbsup:

[Dennis]

I’ve just trialled my Mk II mask, with the clear transparency slots cut out and wow, what a difference. The results are simply astonishing. There is absolutely no doubt about the point of focus and I think this will even beat software assisted focusing.

My next trial (not tonight) will be to try it on “Remote Live View” on the Canon 40D.

Cheers

Dennis

[Dennis]

Wow, wow, wow and wow again – it even works with a TeleVue x2.5 Powermate inserted into the system. Using my DMK21AF04.AS I had no problems achieving perfect focus, despite the somewhat iffy seeing conditions! That’s using the Mewlon 180 F12 with TVx2.5 giving a nominal F30.

What an ingenious design; so elegant and effective – great stuff Pavel, I hope you gain some benefit (hopefully financial!) from releasing this design into the amateur astronomy community.:thumbsup::thumbsup:

Cheers

Dennis

[h0ughy]

Dennis,

you are a legend, you are super sleuth of the year for finding this one

[Dennis]

Here is an animated gif of a focusing run on Fomalhaut (1.2mag) at the F12 prime focus of a Mewlon 180 using a DMK21AF04.AS. Note the jitters are due to my lack of skills in generating the gif and are nothing to do with the operation or functioning of the mask.

WARNING: 1.0Mb file size.
Fomalhaut animation (http://www.iceinspace.com.au/uploads/Fomalhaut.gif)

Cheers

Dennis

[Gama]

This is similar to Rod Wodaski's diffraction spike focusing.
Rods is much easier, and just as accurate as it uses the same method (Diffraction spikes). An example here http://home.comcast.net/~dandkmccrady/Articles/FocusMask/FocusMask.htm can be seen.
No measurements, photocopying etc, so much easier.
Theres plenty more ways to use diffraction spikes for focussing, just need to look. As was found here by Dennis.

Theo

[MrB]

Theo, yep thats true, but It seems with this method that it's easier to tell when its at exact focus.
I have trouble with the three-triangle style Hartmann that I use, can never really tell if the spikes are lined up nicely.
Looks like it's very easy to see with this one, will have to try it

[Dennis]

So far, from the two relatively short testing sessions that I have conducted, my impression is that Pavel’s mask seems more “certain” and “absolute” compared to the on-screen example I have just looked at on the Ron Woadski link provided.

My Mewlon has a 3 vane spider and so all bright stars have 6 diffraction spikes without any mask. Although these diffraction spikes are a very useful aid in getting a good focus, I can still “dither” with the motor movement keys and “interpret” the “best” focus using the built in 6 diffraction spikes.

When I fit Pavel’s mask, I experience something quite different. There is no dithering or interpretation; the spot of best focus just jumps out due to the “moving bar” being located between the arms of the shallow X.

Note that here, I am comparing Pavel’s mask to my “native” diffraction spikes as I have not used the Wodaski Mask and Method.

Cheers

Dennis

[Dennis]

Here is an example using Altair. The central “moving diffraction bar” is either in between the “stationary” shallow “X diffraction bars” or it is not, so the positioning can be quite precise.

Cheers

Dennis

[Gama]

Thats why the chap in my post used circles with a strip thru them instead of Rod's method, so he can remove most of the extra diffraction spikes.

I use this method, and its easy and accurate.
Still, Dennis is on a crusade with this one.


Theo.

[h0ughy]

i am with you on this Dennis, i think this method looks far better then the Hartman mask with diffraction gratings. I downloaded the files off cloudy nights but do not have Corel draw. might have a go at a CAD drawing of this and send it to a CAD/Cam cutter to make it up for the 8" vixen, the 127ED and the 8" SCT and the ED80, oh and the 10" lx200R.

[h0ughy]

arise SIR DENNIS:P and mount your trust steed:whistle:, go forth and vanquish the unbelievers and convert them to Bahtinovism.....................:he lp::lol:

[RB]

Thank you for the info and heads-up Dennis.

Any chance of a pic of the actual mask you made please?

[Dennis]

Hi Dave

So far, I have found that the Bahtinov Mask works at the prime focus of the Mewlon and also with a TeleVue x2.5 PowerMate in the optical train, both visually and with a DMK21AF04.AS as the sensor.

So, even though the calculation as written, of Focal Length/(Range 150-200) = Bar + Space dimension gave me a Bar Width of 6mm and a corresponding Bar Gap of 6mm using a fl of 2160mm, the same mask seems to hold up even at a fl of 5400mm.

At shorter focal lengths of say, 800mm and less, the formula changes to 3xF/(Range 150-200) so the bars don’t get too thin.

Therefore, for your larger ‘scopes, you may only require the largest mask with some means of fitting it to the smaller scopes, rather than a mask for each?

Cheers

Dennis

PS – I drew up my mask in Corel Draw and would be happy to send you a high res file if you want it.

[Dennis]

Hi Andrew

As I appear to have unwittingly embarked on a Crusade, I guess I should continue with the expedition, even though I don’t yet have papal sanction.:shrug:

Therefore, I will grab a couple of photos later today and post them.:lol:

Cheers

Dennis

[iceman]

I love a good crusade :) Can't wait to see the pics, Dennis.

Make me one! :)

[acropolite]

Dave, if you go down that route I'd be interested as well, I expect that the price per unit would be better for a quantity buy. What would be really nice is for someone to write a simple program to automatically generate the drawing.

Perhaps Mike could put it in the resources area.

[Dennis]

Here are some quick pics of the Bahtinov Mask that I made to give an idea of its construction. I drew up the mask in Corel Draw with a bar Width of 6mm and an inter-Bar Gap of 6mm based on my focal length of 2160mm.

The mask was printed onto an A4 overhead transparency which was slightly too narrow, although wide enough for the 180mm aperture of the Mewlon.

I cut a strip of camping mat foam (closed cells) and made a ring which just fitted over the end of the OTA. I then cut the transparent gaps out of the transparency film using a steel rule and a Stanley knife and glued the mask to the foam ring.

Where I accidentally scratched the ink during the cutting process, I used a permanent marker pen to blot out the scratches.

Cheers

Dennis

[sheeny]

Hey Dave,

When you find out how much to get them CAD/CAM cut, can you let me know? I'd be interested in one for my C8 too if it's practical to tack onto your order...:P

Al.

[RB]

Excellent, thanks Dennis !

[h0ughy]

hey Dennis, RB says can you slow up the animated gif -- its too fast for him

[h0ughy]

Hey Dennis – are you going to make the proper version in Teak or walnut finish?

[h0ughy]

sure thing:thumbsup:

[Dennis]

What attracted me to the effectiveness of the Bahtinov Mask was the ability to obtain critical focus at long focal lengths and long(ish) exposures.

When using a PowerMate or Barlow to produce an effective focal length of over 5000mm and exposures of over 1 second, this mask seems to be able to dial in the focus very quickly and unambiguously.

Another scenario for a future test is using it with “Remote Live View” on the canon 40D to assist with DSLR focusing as my set up does not have any software support for this task.

Having said all of that, I must say that the nerd in me had the Bahtinov Mask on the ‘scope last night as I went through my range of eyepieces, pumping the motor focusing buttons just to watch the intra-focal, at focus and extra-focal patterns as they danced in front of my eyes.

Cheers

Dennis

[Dennis]

I’ve just slowed it down to 5fps from the original 15fps. The “bouncing around” is a result of the alignment software not coping too well with an image that is changing so much between frames.

I'm NOT going to align them manually....:P

Cheers

Dennis

[netwolf]

Wow this is just fantastic work Dennis, a great find. I have looked at the thread over at CN and the process looks so straight forward and easy to do.

h0ughy i to would like to now what it would cost to make one, this would be great for my C8 and smaller refractors.

Just a thought i had, and it maybe completely impossible. But could something similar to this be used as a filter at the EP? I mean something that could be attached at the focusing end of the telescope. A 1.25" or 2" filter. I am thinking i guess along the lines of the Knife edge and Ronchi type focusing tools.

Regards
Fahim

[Dennis]

Hello,

Here is an “in-line” animated gif of a truncated segment of the original avi file. I haven’t been able to stretch the histogram so the stationary shallow “X” diffraction bars and the moving “central” diffraction bar don’t show up quite as cleanly and clearly as when viewed on screen at the time of capture.

With the naked eye and a Pentax XW14mm eyepiece, the pattern is as bright as a sparkling diamond!

Cheers

Dennis

PS - Press “Refresh” to run or re-run the animated gif.

[MrB]

Sorry, I read your original post as saying that the circles with strip is Rod's method, I just re-read it and still get that impression.
Anyway, the circle/strip method looks like the spikes line up together in the centre.... exactly where the star is, making it hard(for me) to see when they are nicely crossing each other because the star is in the way.
With the three-triangle that I use, the lining up of the spikes is done away from the star, as you get closer to focus, the lines(which are parallel to each other), will move closer eventually overlaping each other at focus.
Imagine holding clenched fists in front of you with each index finger extended(representing two of the parallel spikes, fists as star), now move you hands closer and closer untill the fingers(spikes) overlap.
Even then though, I find this difficult as its hard to tell when they are exactly overlapping each other.

Pavels method looks great, because it is also done away from the centre, but there is no overlapping, making it easier to see minute movements of focus.

Three-triangle example can be seen here: http://www.iceinspace.com.au/63-187-0-0-1-0.html
Strangely, the examples there don't show large diffraction spikes when not in focus, but I do get them. (even with curved spider)

Anyway, I guess whatever works for you, works for you.

[Dennis]

Just some reminders. If anyone is going to the expense of having one of these made by an external supplier, don’t forget to read the CN post (http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2544487/page/0/view/collapsed/sb/5/o/all/fpart/all/vc/1)– it continues to grow with new information. The formula I used was as follows, for long focal length ‘scopes:

(Bar width + inter-Bar gap) = Focal length/(Range 150-200).

The range 150-200 is a variable denominator to allow for a calculated result that is a “sensible” value such as 12mm, 6mm, etc rather than say, 10.1732mm. I’m not sure how scalable the design is; my bars and inter-bar gaps are 6mm with the main central bar being 15mm wide.

For short focal length ‘scopes (less than 800mm?) the formula becomes:
(Bar width + inter-Bar gap) = (3)x(Focal length)/(Range 150-200).
This x3 multiplication factor prevents the bars becoming too thin.

Don't forget to post your pics!

Cheers

Dennis

[Merlin66]

Netwolf,
If you have a Ronchi grating you don't need anything else. It will give you pin point focussing everytime!
I think the strength of the objective screen methods is that they provide an image to a webcam/ CCD which can then be focused prior to taking an exposure.

To do this with a Ronchi you really need a flip mirror and the screen position at the same distance as the CCD plane. A bit of quick trial and error usually gets it right.

[Dennis]

Hello,

Here are a couple of screen prints of Fomalhaut using the Bahtinov Mask in conjunction with the Canon 40D “Remote Live View” utility, showing the standard view and the x10 zoom setting view. The best focus was very easy to obtain at the x10 zoom setting.

The star used in this example was Fomalhaut (1.2 mag) and the actual on screen view was slightly better than this screen copy. The two “stationary” shallow X bars and the central “moving” bar were easily visible on the Notebook.

Another good focus indicator is being able to view the pair of 3 distinct diffraction spots either side of the single central diffraction spot.

Also attached is the central, full size crop of a subsequent 5 second exposure of Fomalhaut with the Canon 40D after removing the Bahtinov Mask. Scope was Tak Mewlon 180 at 2160mm F12 prime focus. The 6 diffraction spikes are artefacts of the Mewlon’s 3 vane spider. Focusing life just doesn’t get much better than this!

Cheers

Dennis

[AlexN]

Im going to have to make one of them... I looked at them on cloudy nights, and a guy there is selling them for $169USD for my scope ... Forget that!

Looks very very slick.

[h0ughy]

thats why i am looking at getting them done locally;)

[h0ughy]

i attached a file but not sure if i got the formulae correct. if it is correct then there is a big difference between scopes and grid sizing


Edit---> for correct formula go to post 50 (Al's posted a corrected file there)

[RB]

I can't open the file Dave, is it complete?

[AlexN]

works for me...

[AlexN]

I wonder how much it would cost to get one lazer cut out of 2mm thick AL sheet..

Oh, I'd need a couple of them wouldnt I... one for prime focus @ 2800mm, one @ F/6.3, and one for planetary imaging @ 6~7m focal length.... could probably do without the one for planetary... but even then, 2 of them for F/10 and F/6.3 respectively would be expensive to get made as one off's

[Dennis]

If some enterprising metal (or even plastics) fabricator has a laser cutter or similar, they could probably cut out aluminium (plastic) discs with the mask patterns at a reasonable price locally?

Maybe Ron at Sirius Optics, Underwood has some connections to local workshops?

Cheers

Dennis

[AlexN]

yeah plastic would do...

Mine needs rather thin bars/inter-bar gaps... 5.5mm @ F/6.3 and 8mm @ F/10.

Plastic would probably be a better option that Aluminum, as the idea of putting some thing that heavy and solid next to the corrector plate scares me a little bit.

[h0ughy]

i am getting a rough quote from a laser cutter from Victoria - will keep you posted. Dennis did you have a look at the rough calcs i did - are they correct assumptions or am i off my tree:help:

[AlexN]

David, the calcs in your excel sheet look good to me, although in the "formula" bar, its got D3/(150-200) - which in excel means D3 (focal length) divided by 150 minus 200) I simply removed the 150-200 part and experimented with values to give me fairly round numbers...

[Dennis]

I have an idea. I’m going to design and build a low cost motorised version, with adjustable components, for on the fly customisation of bar/gap widths and angles. :whistle:

Each bar will be controlled by its own stepper motor and angle adjuster so that I can adjust the bar width and gap, as well as the bar angle, to suit the ‘scope/sensor of choice on a per session basis.:P

There will be a hand controller so you can just punch in your ‘scope model and optical configuration and the mask will then self adjust. There will be the ability to store up to 100 Favourites in a custom library. It will be light weight and cost less than the Apollo Program!:lol:

Cheers

Dennis

[AlexN]

LMAO! nice dennis.. I'll be the first to pre-order!

[Dennis]

No, no, no…it is not 150 minus 200! Good pick up Alex.

The denominator is a range where you choose a value between 150 and 200 so that the bar width comes out as a reasonable number.

Cheers

Dennis

[h0ughy]

:rofl:the sad part is dennis - i was thinking about that and how it could be implimented - a separate mask is a lot easier:thumbsup::rofl:

[h0ughy]

Ahhh me too literal ----can you ammend formula and upload

[sheeny]

Dave,

The formula at the moment is not correct. Instead of D4/(150-200) I believe it should be say D4/x , where x is a number between 150 and 200.

I would suggest doing two calculations, one for D4/150 and one for D4/200 and then picking an integer size between the two...

Clear as mud?:P

Al.

[sheeny]

Here ya go Dave,

Have modified your file with my interpretation...:P:whistle:.

I notice the calculation is for bar/gap spacing combined. So presumably that is the critical measurement not the width of the bar or the width of the gap... soooooooooo... has anyone thought of modifying a cake rack to do this?:shrug: A wire grid would maximise the light into the scope...

Just a thought...

Al.

[RB]

From what I see so far, Dennis' idea of the printed overhead transparency is the simplest and most cost effective method, especially for multiple F.L and it seems to work perfectly.

:thumbsup: :)

[h0ughy]

thanks Al that fixes things up a bit (a lot really) -- happy baking:P

[h0ughy]

The C8 and the LX200R 10” would use the 13mm spacings while the vixen R200SS would use a 5mm spacing – the same for the 127ED. The ed80 would probably be a 10mm spacing

[netwolf]

Maybe i missed something but would not adding a barlow change the width requirement?

[AlexN]

yeah at every different focal length you need different spacings.

[h0ughy]

bung it in the formula - it changes the focal length that s for sure;)

[netwolf]

So you would need one mask per barlow setting.
But according to Dennis it does seem to work on his scope with a Barlow. So question is how sensitive is this to the change in FL?

Regards
Fahim

[MrB]

There's a simpler version, but would have no angle adjustment.
Use two of the same masks stacked one on the other, move one to the left(or right) and the spaces will close up.
Using this, the bar width would be adjustable, but their pitch wouldn't change, is that an issue?

Edit: actually, the more I think about it, the less it's gonna work. The spaces would be shifted off-centre as the bar width is adjusted.... unless.... both masks move simultaneously, one left and one right equal amounts... hmmm

[sheeny]

After thinking a bit about the "modified cake rack" option, I now think I might try the "String Art" option...:P:whistle:.

Timber frame with nails at the appropriate places for the prototype and some cotton... (maybe use different colours so looks pretty hanging on the wall :lol:). If it's a success, the real one would be a timber frame and fishing line through holes (just like stringing a racquet:P).

Turn that old photo frame into something useful...:rofl::whistle:

Al.

EDIT: Hmm I'll have to think about that a bit more... I just realised my initial string art idea would just produce lots of diffraction spike and not aid focusing:rolleyes:. But watch this space...;)

[Dennis]

Hello,

Here is a comparison using a single Bahtinov Mask for the following configurations:

Mewlon 180 at F12 prime focus, fl=2160mm (DMK21AF04.AS).
Mewlon 180 at F30 with x2.5 PowerMate, fl=5400mm (DBK21AF04.AS).
Mewlon 180 at F36 with x3 Barlow, fl=6480mm (DBK21AF04.AS).
Mewlon 180 at F12 prime focus, fl=2160mm (Canon 40D Live View x10 Zoom)

The same Bahtinov Mask was used in each instance; that is, the one calculated at 2160/180=12mm giving a 6mm Bar and a 6mm gap.

Cheers

Dennis

[RB]

Thanks for the comparison Dennis.
As I suspected, the same spacings would suffice at different FL on the same scope IMO.
I wouldn't be too fussed and would only make one mask per scope.
I think it would get you to very near perfect focus each time.

Anyway I'm looking forward to making up a mask for my scopes and trying it out with the 20Da.

[AlexN]

Should be ok... How do you think it would go for planetary imaging, with say, a 2800mm focal lenght + 5x powermate (lets just assume that we ever get seeing nice enough for that... thats an increase of 12,000mm in focal length or there abouts... Do you think it would still be effective then?

[Dennis]

I haven’t investigated the maths behind Pavel’s equation (I cannot read Russian!!). Also, the CN thread doesn’t seem to have gone into any mathematical analysis of the equation in terms of wavelength of light, focal length, diffraction patterns, etc., so I am limited to empirical testing and so far, a single mask seems to have worked within the range that I have experiment with.

I’m not crash hot on the mathematics of diffraction and how different instruments behave so perhaps it is a blessing that I can’t read Russian! LOL!

The x2.5 and x3 Barlow shots required exposures of around 2 seconds IIRC.

Cheers

Dennis

[GeoffW1]

Yea!! Plaudits to Dennis and IIS!!:einstein:

I just made up one of these in thin paper from a scaled image in MS Word, with a sharp blade and a steel rule, and it works beautifully, looks like the image of Altair in Dennis' post. Great information :thumbsup:

It is (accidentally, I did not really try) optimised for the Skywatcher f1000, being 3 + 3 = 6mm, and the electric focuser I just fitted to it helps a lot as well.

Cheers

[jerry3672]

Hi all, I'm new to your forum and would like to explain what is going on with www.spike-a.com (http://www.spike-a.com)

The focus mask was offered because of the interest people had at cloudy nights to have me make them masks. So John helped me by placing the spike-a web site up so people that could not make a mask had somewhere to purchase one. We have attempted to get Pavel to allow the use of his name on the mask. So far we have no response from e-mails. I went to an lawyer to find out what the correct royalty should be and how to send Pavel the credit he deserves. If anyone here speaks Russian, I sure could use the help communicating.

Jerry

[Dennis]

Hi Jerry

Welcome to Ice In Space and thanks for the details provided.

I do recollect reading one of your posts on CN along the lines of “what a surprise Pavel will get when the Royalties start flowing in” so I figured you were pursuing some kind of arrangement to recognise his contribution!

Are you considering setting up a distribution outlet in Australia?

Cheers

Dennis

[h0ughy]

OK I got the quote back from an Australian Laser Cutting manufacturer (Lightning laser cutting in Victoria), for a 1 off made to test it will cost me $82.50 a pop. Obviously a single one is dear but multiples are a lot cheaper. This is based on a dia 200mm and a central bar of 20mm with 20mm added to the outside, and 250mmm for the larger one. Looked at two different materials – black acrylic and stainless steel 304 grade. I haven’t looked at a way to fit these to the scopes, although a 90degree clip screwed to the template with a plastic screw to attach to the scope did come to mind, or even a Velcro attachment? As didn’t get a quote for the 10mm spacings, as I thought at this stage that was pointless, but I would say it falls between the two. Smaller diameter masks were not looked at yet, but where they are less in material the same setup costs would be involved, so I don’t envisage them to be much cheaper. Postage is not included in these prices, obviously there are economies of scale – so quantities of more than 5 will bring the prices down – but then it all comes back to how much do you value a gadget? For me I think this will be priceless, especially considering all the crap images I have taken off late that are out of focus.

1 Diffusion grating dia 200mm, 5mm slots from 2.0mm black acrylic $82.50
1 Diffusion grating dia 200mm, 5mm slots from 0.8mm SS304 $82.50
1 Diffusion grating dia 250mm, 13mm slots from 2.0mm black acrylic $82.50
1 Diffusion grating dia 250mm, 13mm slots from 0.8mm ss304 $82.50
5 Diffusion grating dia 200mm, 5mm slots from 2.0mm black acrylic $28.60 143.00
5 Diffusion grating dia 200mm, 5mm slots from 0.8mm SS304 $33.00 165.00
5 Diffusion grating dia 250mm, 13mm slots from 2.0mm black acrylic $27.50 137.50
5 Diffusion grating dia 250mm, 13mm slots from 0.8mm ss304 $31.90 159.50

[h0ughy]

Hi Jerry and welcome to IIS. I have read the happenings on CN. No one i know speaks russian, but would a translator like bablefish (http://babelfish.yahoo.com/)help?

[Dennis]

Hi Dave

Before you and others commit to an order, or multiple orders, I would recommend that the team perform a design review to validate the design, dimensions, proofs/templates, materials, fixing methods, anodising, flat black paint finish, etc. to minimise any surprises or glitches.

As a minimum, an independent person(s) should:

Review the measurements to ensure that the calculations have been performed correctly.
Review the proofs/templates prior to submitting them to the cutters to verify that they are correct.
Discuss having tabs or ears included at the outer edge which can then be bent over (SS only) for fitting.
Discuss the finish as SS is a bit too shiny?
Discuss any other issues that may enhance the performance and use of this device.

We have already seen how easy it is to misinterpret the denominator in the range 150 to 200.:whistle:

As they say, measure twice, cut once.;)

Cheers

Dennis

[RB]

I suspect that the mask 'spacings/gaps' is more a function of the scopes aperture (diameter) and native F.L.
If you design it for a specific aperture it should work within the limits of adding a reasonable 'barlow' to that scope.
What you will notice IMO is different size spikes but using a barlow within reason on the same scope should still yield a usable 'focusing image'.

What I meant when I said multiple masks for multiple F.L's was having to make individual masks for individual scopes with v.different apertures/native FL.

So I believe I'll only need one mask for my 5" and 6" APO and one for my 12" SCT.

[AlexN]

brilliant.. thats pretty much what I wanted to hear..

I'll see if I can find somewhere local to make one up for my M90 and my C11...

If not, Dave might have more luck and If possible I'll tack them onto the order.. :D

[Dennis]

Hi Geoff

All credit goes to Pavel for sharing his design, demonstrating the noble and constructive use of the internet.:thumbsup:

Like you I was enthralled by how effective the Bahtinov Mask is when used visually. I must confess, I spent several minutes working through my eyepiece collection, just pushing the motorised focuser buttons and watching through each eyepiece as I cycled through the extra-focal, at focus and intra-focal patterns!:whistle:

My wife was quite impressed too, although somewhat bewildered at how I could just sit there engrossed in the whole focusing experience thing.:shrug:

Cheers

Dennis

[h0ughy]

nothing like a few playthings to keep one amused for hours - my son can play in the sand pit up the back for hours - and in a cardboard box for longer:whistle:

[h0ughy]

i fully agree Mr Simmons - I haven’t jumped in and ordered anything - just getting a ballpark guesstimate:whistle: to see whether it could be feasible:whistle:

[Screwdriverone]

Looks very good Dennis,

I have just knocked up a three triangle hartmann mask from the instructions on the Paul Russell, IIS projects pages and it works like a treat!

I drew it up on a piece of A4 (my scope is only 130mm after all) and then scanned it, painted the white areas black in photoshop, printed, laminated it, cut out the triangles and attached it with velcro to the aperture (I have a ring around the opening for a light shroud using vinyl from Clark rubber)

Focusing is now a complete DODDLE! Woooohooo!

Thanks for the inspiration, I might endeavour to make one of these later on, however the basic three triangle one took me all of 10 minutes and a compass/ruler, no CAD required.

Cheers

Chris

[Dennis]

Cardboard box….he is lucky…. Well we had it tough. We used to live in a small shoebox in the middle of the road.

We used to have to get up out of the shoebox at twelve o'clock at night, and LICK the road clean with our tongues. We had half a handful of freezing cold gravel, worked twenty-four hours a day at the mill for fourpence every six years.

But you try and tell the young people today that... and they won't believe ya'.

Cheers

Dennis

PS – Here is the 4 Yorkshire Men (http://www.youtube.com/watch?v=Xe1a1wHxTyo)sketch by Monty Python

[Screwdriverone]

Ahh yes, but when you got home, your father used to thrash you to sleep with a broken bottle.....

AND you were thankful!!!

;)

I thought it was you who lived in a paper bag in the middle of a septic tank?

Might have been Mrs Johnson who lived next door to the cheese shop? - oooh dont get me started on that one.....

;)
Chris

[MrB]

I never bothered painting my 3-tri Hartmann (tri-tri?, tri^2(trisquared)? :D)
Just left it as white card, haven't had a problem with it.

[AlexN]

That 4 Yorkshire Men sketch is possibly one of the funniest Monty Python sketches ever done! :) Hilarious.

[h0ughy]

:rofl::rofl::rofl::rofl::rofl: your a national treasure Dennis:thumbsup:

will looka t the design in detail over the weekend and look at how to attach to scopes - to guage sizes and shape for design. the tab idea might work:thumbsup: as for the central bars/circles not too sure on those:shrug:

[sheeny]

:lol: Got to love Monty Python! ... and so much of their stuff is applicable to all sorts of situations.

Poking around in the shed last night I found some lengths of 20x20x1 angle aluminium and some 12x3 aluminium bar. It might take me a while to fit it in around all my other projects, but I'm going to try fabricating a mask to fit the C8 based on an aluminium frame and fishing line for the "bars".

I figure since fishing line works to make diffraction spikes when applied across the objective, it should work OK for this job as well. It is after all the edges that produce the diffraction, not the width of the bars. That being the case a fishing line mask should allow maximum light into the scope which just might make it usable on fainter stars:shrug: or with less changes to camera settings.

Al.

[Dennis]

Hi

I just had a browse of the Russian Astronomy Forum (from a link in the CN post) and noted that there appear to be several variants of the mask.

The one that I have drawn has a full width (solid) horizontal bar.
One variant has a gap, not a bar as the central horizontal feature.
Other variants have a solid isosceles triangle on the angled side.

Hmm, I wonder which is the optimal design? All of them?

Cheers

Dennis

[sheeny]

Intuitively I would expect the longer the total length of edges in one direction the stronger the diffraction spike in that direction... sound feasible?:shrug:

Al.

[jerry3672]

I am a member of the Russian forum hosted by Pavel. I asked for permision to call the mask by his name.

The only plans to sell the spike-a mask in Australia is by shipping from the US. Info can be found on the web www.spike-a.com (http://www.spike-a.com)

[h0ughy]

Jerry, i have a few questions:-
- according to the formulae the grid pattern widths change with the focal lengths of the scope - the product you seem to be selling does not look as though you can take that into account but rather have to accept a predetermined grid width that you have setup for manufacturing - IE with the 10" Meade it is 13mm, but for my R200SS vixen f4 it is 5mm grid spacings? Your web site has no options for choosing a grid sizing - just the outer diameter of the grid mask?:shrug:

[AlexN]

Thats a good point David, I hadnt noticed that... Also, if you have an F10 8" with a 6.3 F/R.. according to the formula it should require different width bars, but no options for such spacings.

[jerry3672]

My math is not the same as the Russian site. I use the clear aperture and not focal length to determine the cutout and spacing. I am not concerned about hitting the 3rd, 5th, and 7th order spectrum as much as getting the maximum light gathering power of the scope. Very simple math- For the horizontal cutouts just take the clear aperture and divide by 35. This will give you a center cutout and 8 more in each direction. So for a mask that will fit an 8 inch SN type scope the math is 8/35= .228 inches. By the way this is the 10 inch mask. For the angled side- multiply the cutout on the horizontal side by 1.05. so the example is .228 X 1.05 = .240 inches.

I think too many people are getting caught up in the math formula that hit the spectrum order for a particular focal length. My test shows that is is not as critical as good straight lines and a mirror image top to bottom and left to right.

For those that just want to hit the spectrum and split the color band, then by all means, take the time to calculate your focal length. But if you just want an easy way to get the best focus fast-- Then keep it simple.

I have posted this to Pavel and his group as a suggestion. And now on to you nice people.

[jerry3672]

I did get translating software and have contacted Pavel. He said we could call the focus device a "Bahtinov Mask". I made my intentions to share any profits with him.

I hope nothing gets lost in the translation. i don't want to offend someone that has offered such a great tool to amateur astronomers all over the world.

[h0ughy]

Yep the last thing you want is a mistranslated word;)

DId you trial many variants of this design before you settled on your version?:shrug:

[Dennis]

Hello,

The seeing was quite poor last night, hovering around 4-5/10, so I thought I’d have a play with my home made Bahtinov Mask, as the detail on Jupiter and even the Moon at prime focus was washed out and watery.:(

Here is an image of a grossly over exposed Jupiter and 3 of his Galilean moons at the F12 prime focus (2160mm) of the Mewlon 180. With the correct exposure for Jupiter’s disc, there were 3 Jovian discs – the “real” one and a pair fainter facsimiles either side from the mask.

You can just make out the diffraction pattern from Jupiter’s washed out discs (all 3 of them due to the mask!)

Cheers

Dennis

[iceman]

Interesting, thanks Denis. I was wondering how it would work on the gas giants.

I can see that focusing on the galilean moons would be easier.

[jerry3672]

I did my share of testing. They all worked except for one. I sent a design to John for his 80mm. We had the bright idea that you could print the pattern on a clear transparency and it would work. The transparency changes the focus just like adding a filter to the eyepiece. The spikes were dull and not in the center when the ccd was in focus. My last design is what i posted here. It is very forgiving to the focal ratio. Still the bigger scopes put up a brighter pattern. I can focus my dsi with my sn-8 down to a mag 6 star with no problems. This is nice when your target is not near a bright star.

[h0ughy]

ok just drew up a couple of templates - one for Alan Meehan's ED100 and the other is a test for the C8 and the Vixen R200SS - just to see if it will work on both. Warning - the ED100 is on a4 paper and is at true size - the other is A3 paper size for the 8" scopes. I havent measured the outer diameter of the scopes yet so a little bit of poetic license was used for the outer area of the pattern. See what you think?:shrug:

this version of the design took components from dennis's, a few of CN and one on the russian site, of which I included the image of here. I am not sure as to the benefit of the central mask circle, but i am sure there is an explaination as to why it might be needed?

[h0ughy]

thanks for the reply:thumbsup:

[h0ughy]

OK I have done up a template for the 127ED "meade" look alike scopes and included the CAD drawing and PDF files as well.:screwy:

[wunder]

Hi all- I had to jump in on this one. I'm Jerry's business partner, John Wunderlin.

You can't use Jupiter very well to focus- I saw the same thing in my testing- it has to be a pinpoint of light, though on the Russian forum I see someone was able to use lights on a distant building in daylight that worked pretty well.

The other item related to this is that collimation affects the pattern. If your collimation is off, the pattern is distorted and my not provide proper focus, though if your collimation is off, you'll have additional imaging issues :)

[RB]

Welcome to the forum guys, good to have you here.

:welcome:

[Dennis]

Hello,

With the seeing so poor last night, 4-5/10 at best, I decided to just have a play around with my home made mask and whacked a x4 PowerMate into the Mewlon 180 F12 to give me an efl of a whopping 8640mm at F48.

Using the SBIG ST7 CCD camera I picked up a mag 5.16 star and was still able to focus using the mask, although I had to push the exposure out to 2 seconds. What I did notice, is that the focusing assist function in CCDSoft (the ST7 camera control software) gave me a slightly different “best focus” indication compared to the mask.

So far, I can only think of 2 reasons for this:

My mask is made from a cut out overhead transparency and is therefore floppy so all the diffraction elements do not lie in a single plane.
The weight of the ST7 CCD camera and CFW8 filter wheel dangling on the end of the 2” x4 PowerMate was causing the motorised focuser (Moonlite) to flex slightly so the plane of the CCD chip was slightly angled.

The effect is more noticeable in the right image of this pair.

Incidentally, I was still able to use the CCDSoft focusing assist function with the mask fitted, to find the best point of focus due to the sensitivity of the ST7 even at an fl of 8640mm at F48. Also, having the mask fitted made the overall focusing routine so much more efficient in terms of converging quickly to the optimum focus point.

So, even despite some quite poor seeing conditions, the mask was able to perform very effectively with 2 sec exposures at F48 – an impressive result for a piece of cut-out overhead transparency!:thumbsup:

Cheers

Dennis

[Dennis]

Hi John

Welcome to Ice In Space – it’s nice to have the business owners providing their insight into this device.

The comment about the diffraction pattern seen from Jupiter’s washed out disc was posted as an observation rather than a recommendation for focusing, as I suspect most amateur astronomers are aware of the requirement for a point source of light, although the Galilean moons (extended discs) do serve as very useful indicators to confirm focus.

Cheers

Dennis

[h0ughy]

Dennis,

I don't know why I didn't think of it earlier but I went to an art supply place this afternoon and purchased some black 5mm foam core board. two A3 size sheets were about 8 dollars. it is light, black and you can easily cut it with a scalpel. I have asked Alan if he would be able to make a screen print up for the larger sizes to print directly on the larger (8" and above) masks - then cut it out. Hopefully this will work.:shrug::help: that way the cost for the baoard is under $10, easily made with some TLC and a bit of time and hopefully specifically designed to the scopes requirements. What did you think of the alternate design?

[Dennis]

Hi Dave

I am inexorably being driven to a laser cut metal solution right now, in order to spend less time focusing and more time imaging. Having experimented now with the following materials and designs:

Overhead transparency (gaps not cut out)
Overhead transparency (gaps cut out with Stanley knife and steel rule)
MDF 3mm thick cut with a router (phew!)
Black plastic computer binders (gaps cut out with Stanley knife and steel rule)


I am left with the following conclusions:

Floppy material, such as overhead transparency and black plastic is likely to stretch, sag and not lie in a single plane.
Materials such as MDF, foam core board, etc may absorb water and over time, become frayed around the edges.
The process of rapidly converging to and locating precise focus is such a key element that it is worth obtaining and using a well manufactured, stable and rigid mask.
Ideally, it is best to remove all “easy” variables under your control, such as a sagging mask, furry edges, etc so that you can concentrate on other matters such as eliminating flexure in the focus train, etc.


So, at the moment, I favour either a stainless steel or aluminium thin sheet mask, laser cut for accuracy and cleanness of cuts. I enjoy tinkering so don’t regret the time I have spent just playing around with various designs and implementations, but I am strongly leaning towards a laser cut metal solution right now, in order to spend less time focusing and more time imaging. I’ve had my fun and now its time to get more serious! :whistle::lol:

Cheers

Dennis

PS- I suspect that those with permanent set ups, temperature controlled focusers that can be indexed, etc. may be scratching their heads at these shenanigans! Oh the joys of setting up and tearing down each night, as well as manually pushing motor focus buttons. LOL!

[h0ughy]

I was told that aluminium absorbs the particular wavelentgh of light the laser uses and because of this they have to turn up the wick just to get it to cut - which then over heats the aluminium then it does not give you a clean edge. That is why they suggested the Stainless steel. I am leaning more towards this and if we can get a design secured and tested then have the CAD drawings made (i can do that) then send it off for a final quote and get quantities. Probably ask them for a black non reflective finish to be applied to it - if not then spray the bugger with some flat black spray paint:thumbsup:

was the design I did worth following up or have you a better design?

what I thought of was to get the part to fit inside the dew shield and be supported by foam spacers to keep it off the optical train. then its flat, easy to install and remove. if we can agree on a design - and get some punters in on it - maybe enough for each standard size for the more popular models you could get an economy of scale for the manufacture - if not then they are $82.50 a pop!!

[Dennis]

Hi Dave

Thanks for your good work to date.:thumbsup:

SS sounds good to me and I would certainly add my name to the list for two masks:

1 for the C9.25 and Mewlon 180 (6mm bars).
1 for the Vixen 4" refractor and Canon 400mm F5.6 lens (3mm bars).

I would order the larger size for each and then insert a home made foam ring to reduce the mask for the smaller tube in the OTA pairs above.

So far, my testing has led me to believe that I can use one mask per OTA either at prime focus, barlowed or with a reducer/corrector. Also, $82.50 doesn’t sound too exorbitant for such a crucial task as focusing, which we may perform several times per session over the life of the OTA. Worth every penny!

Cheers

Dennis

[h0ughy]

OK then - send me your dimensions and I will draw up the template in CAD to send off - after i test some more;)

do you like the template or do you prefer the complete striated version of your own?

[jerry3672]

Dennis, my testing has the same results that you see. One mask per OTA.

What currency is the $82.50?

I used 3003 Aluminum because it less likely to crack over time. Also cutting on a water jet will not over heat the base material like a laser. I was afraid that the small cuts for the 80mm scope would end up breaking if we didn't use a high grade of aluminum. A good quality Bahtinov mask will last more than the life of your OTA.

[Dennis]

Hi Jerry

The $82.50 quoted by Dave (H0ughy) is in Australian $. However, this would not be for a finished product such as yours, anodised, with fixing tabs, etc.

Until quite recently, we have enjoyed an almost $ for $ parity with the US$ but in the last couple of weeks, the A$ has fallen to around 81c $US making purchases overseas a bit more expensive.

As you have noted, there are hidden matters that begin to emerge when one begins to investigate detailed design, materials selection and fabrication methods.

Cheers

Dennis

[Merlin66]

Looking like a great idea!
My 2c
The central "disk" is there I think because it can be... this areas is shaded by the secondary ( in an SCT); when placed on a refractor it would help by brightening the first diffraction ring.
A couple of velcro tabs would be very easy and versatile.

[Dennis]

Hi Dave

I just downloaded the zipped CorelDraw files from the uba-post on CN which appear to be some of Pavel’s original designs.

The zipped file contained x3 CorelDraw files and when I opened each one, the centred, middle horizontal element was always a solid bar (not a gap) consistently across all 3 files. I also note that John & Jerry’s Spike-a™ (http://www.spike-a.com/) has a centred gap, not a centred bar.

Correction:
The x3 CorelDraw files in the zipped package do indeed show a blank or gap rather than a solid bar, as the central horizontal element on the left hand side of the mask. Incorrect observation due to reading a thumbnail size image. Upon opening the full size drawings in Corel Draw, it was clear that the central element is a bar.

I suspect that in real world terms, there will not be a difference. However, as we can faithfully draw the mask and have it cut to Pavel’s original design I see no reason not to follow it. That is, keep the central bar a solid bar.
What do you think?

Cheers

Dennis

[jerry3672]

When in doubt go to the original. Here is a link to Pavel Bahtinov's forum. it is in Russian.

http://www.astronomy.ru/forum/index.php/topic,10421.40.html

You see Pavel has a center Cutout just like my variation. The most important fact is that the slots are a mirror image of each other in respect to the centerline of the OTA.

[Dennis]

Hi Jerry

Thanks for that. I’ve just finished reading all 16 pages on Pavel’s Russian Forum in the post dedicated to what has now become the Bahtinov Mask. There are some bright boys over there for sure.:thumbsup:

The Babel Fish translation wasn’t too painful to follow!:whistle:

Cheers

Dennis

[h0ughy]

http://astrojargon.net/maskgen.aspx this was posted on the russian ste

[TrevorW]

So guys where can I buy one at a reasonable price to fit a 80ED with a dewshield OD of 110mm

Cheers

[Dennis]

Hi Dave

I’ve e-mailed you the x3 Corel Draw files that I found on Pavel’s (Russian) Forum from the link on CN.

I also tried the “Generator” from the http link on CN but my installation of CorelDraw V12 would not open the .svg file generated by that cool application, although I have been able to open other .svg files?

Cheers

Dennis

[Dennis]

Hi Trevor

Unless you want to fabricate your own, the only commercial source of these that I know of is from John and Jerry at Spike-a™. (http://www.spike-a.com/)

They have a nice description on how to order for the correct size.

Cheers

Dennis

[multiweb]

Actually I made one for my ED80 and also for my 5" celestron. I have a template that is easily scaleable. Here's a focus run I did tonight around M17 :
http://www.multiweb.com.au/astro/focus_run.gif

and one unprocessed sub at http://www.multiweb.com.au/astro/one_sub.jpg. This was taken next to a 3/4 moon and a thin haze of clouds. (FWHM was approx 6.8" - 7") so far from the best seeing conditions. Nevertheless, considering, the result was pretty spot on.

I think the mask is working great. Saves me heaps of time.

[citivolus]

I seem to have come rather late to this party, but will be interested to compare this mask to one of my own Hartmann/diffraction mask designs that I've been working on periodically for the past year or so. I had investigate laser cutting from ABS and other materials as well, but had mostly given up on developing it commercially due to the large number of aperture sizes that would need to be created and high initial production costs. I had also considered just targeting the refractor market, due to the lower number of sizes it would take to reach a large percentage of the market.

I've kept the design to myself so far to keep someone from exploiting it commercially before I was ready for market, but non-commercial use would have been encouraged. I've built prototypes of my current generation for a C9.25 and a Megrez 90.

Dennis, is there any chance I could stop by some time and see your mask in action? I'm quite anxious to see if this trumps my designs, and if I should toss mine in the bin :) I could throw a basic version of mine together for your Mewlon if I could get the OTA inside diameter, mirror diameter, and central obstruction size.

I'm particularly interested in limiting magnitude, overall brightness, critical focus zone, and ease of use, especially when used with live view. I'm not saying mine is better than this design, just that I'd love to compare the two side by side.

I may just print my own later today to try out, if the kids will leave me alone for long enough :)

Regards,
Eric

[Dennis]

Hi Marc

Glad to hear of your success, my home made Bahtinov Mask has certainly delivered the goods so far.

I had a look at the sub frame of M17 and noticed that on the brightest star with the 4 diffraction spikes, the spikes seemed off-centre to the star disc? These spikes can’t be from the ED80 or C5?

Great image by the way!

Cheers

Dennis

[Dennis]

Hi Eric

I’ve PM’d my contact details so we can hook up some time for a test. It would be very interesting to see your design too. I’ve just built another prototype as I found some 6mm black tape in my wife’s sewing box.

It was somewhat easier laying down strips of 6mm tape and gluing them than cutting 6mm gaps in various materials.

Cheers

Dennis

[multiweb]

Correct. Forgot to mention it. This was taken at prime focus on a celestron 5" newtonian . QHY8 1min exposure. I did an 80mm mask for the ED80 and one 160mm for the celestron.

[Dennis]

Thanks Marc – I assumed the C5 was an SCT!:doh:

Cheers

Dennis

[citivolus]

Thanks Dennis, I'll get back to you some time after this weekend as I'm on call through Tuesday so I can't stray far right now.

I have two designs I'd like to compare to this mask. One of them, upon inspection, functions using a very similar mechanism to Bahtinov's design, however his will likely work on dimmer stars due to it only masking about 50% of the light.

One difference that I am speculating on is behaviour during poor seeing. I find that the design that is similar to Bahtinov's dances around a lot due to seeing, while my other design is much more stable with the trade-off being slightly less precision. I'm not sure how his design handles seeing, given I have not tried it out yet.

I'll PM you the designs and a focus movie to look at.

Suggested prototyping materials:

A4 card stock, two sheets wide with some overlap, joined using spray adhesive. You can print to the cards and then cut with a sharp knife after joining them.

3mm or 6mm Depron foam (or the card stock backed foam mentioned elsewhere in this thread) can be used in conjunction with the above to add some rigidity to the design without adding significant weight. The spray adhesive works well here. Sharp corners can be a bit harder in Depron, however if backed with the card stock, precision cuts in the Depron become less critical as the card stock will mask the light anyway.

An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.

Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect :P

Regards,
Eric

[jerry3672]

I think you will find that the seeing conditions causing the Bahtinov mask to "dance" is more useful to capture the best average focus over a mask that is not a sensitive. My method is to focus on or near your image target. Use the same camera settings to focus as you plan to image, this will yield the best average focus in your image. In my opinion this is the best feature of the Bahtinov mask. Focusing on dim stars.

Collimation will affect the mask, just as it affects the final image. Rotating the mask 90 and 180 degrees will test collimation. However, if your mask is not centered or lines are not a mirror, then this will also magnify the error.

The best method to test various designs is to use a deep sky target image and measure the results with something like CCD inspector. Look for the best FWHM value. After all, this is intended to be used for imaging.

Lots of people have claimed the device works just because they see diffraction spikes that move with the focus knob. Just my 2 cents worth.

[Dennis]

Hi Guys,

I think that I have bottomed out the reason for the slight focus variation in my original A4 transparency cut-out, Bahtinov style mask.


The mask wasn’t glued around the entire circumference as the narrow dimension of the A4 sheet of transparency did not quite stretch all the way to the edges and the unsupported edges introduced some flexing into the mask.
I inspected the templates I drew and discovered that the “snap to” function had aligned the bottom and top edges of the bars at 20° and not their centre lines, so as Jerry comments, the two sides were not a mirror reflection.
The focusing error did not appear to be noticeable by eyeball inspection. It only became apparent when I fitted the SBIG ST7 and used the CCDSoft focusing routine which I includes a FWHM and maximum value function.

I have since made a new mask from A4 Transparency that fixes the above issues and I am just waiting on an opportunity to test it out.

I have found that with poor seeing (I had some 4-5/10) the Bahtinov mask appears to still perform very effectively, even with exposures of around 2 secs to average out the seeing.

Cheers

Dennis

[sheeny]

Had an afternoon of heavy rain today so I dabbled about in the shed and produced a prototype mask from aluminium and fishing line. It's 250mm square inside the outer flange of the aluminium so it will slide snugly over the C8 objective with the dew heater in place.

The frame is made from 20x20X1 Al angle. The two cross pieces in the middle are from 12x3 Al flat. Filaments are 12mm apart. The angle differs from a true Bahtinov Mask a little bit - I made it 1:3 to aid marking out.

I ballsed up the threading of the mask though:rolleyes::doh:. The filaments should come in the front of the outer frame and the back of the cross bars... but I'm not about to re-string it yet, I'll try it out first.:whistle:

Al.

[Dennis]

Hi Al

That looks like an interesting design; I would be very keen to see the results. Good luck with the weather and subsequent testing.

Cheers

Dennis

[sheeny]

It'll be an interesting experiment, for me at least. Maybe I'm over simplifying things a bit, but I'm interested to see if it will do the same thing. If it works, it should maximise the amount of light entering the scope so focus should be possible on a wider range of stars:shrug: or less change to camera settings... maybe...

Jerry's comments about symmetry could be a cause for concern though:P. As you can see from the photos the angled filaments are not quite a mirror image (bit of a fitting error there:whistle:).

I'm keen to test it and see what it does though.

Al.

[AlexN]

that looks like it would work.. I'm interested to hear your results on that..

How difficult is it to make clean cuts in foam core board? I think that would be the cheapest way to test many of the different designs, then once you've settled on the one that works best for your setup, take that foam core mask to the local lazer cutters and get one cut out of acrylic or delrin..

[Dennis]

Hi Alex

In terms of constructing prototypes, I’ve found the most flexible and efficient technique so far (for me) is to draw up the templates in a drawing application, such as Corel Draw and then either:

Print off the design on an A4 transparency and then cut out the gaps with a sharp Stanley knife and steel ruler, or
Print off a mask outline on plain, white A4 paper and use strips of tape to create the mask, aligning the strips of tape to the printed out template.

I have used the magnetic tape out of an audio cassette and some 6mm tape from my wife’s sewing kit. This way, you only measure once at the drawing stage and don’t have to mark out the work.

Cheers

Dennis

[allan gould]

Have been very interested in this thread - thanks Dennis et al.
I finally managed to make one my self and will test it tonight on the 5" to see how it performs. Should be very interesting.

[iceman]

I'd love to see someone make a template of some sort for a variety of scopes of varying apertures etc.

eg: ED80, 6", 8", 9.25", 10", 11", 12" etc

[Dennis]

Hi Allan

Some of the nice features about the Bahtinov Mask are that you can quickly establish which side of focus you are on and then converge to focus quite rapidly.

For my SBIG ST7, I usually tweak the final focus using the CCDSoft focusing routine. For the DMK and Canon 40D, I just use the Bahtinov Mask as I don’t use software assisted focusing with these cameras. Remote Live View on the 40D is a great asset!

Cheers

Dennis

[AlexN]

i've found focusing the qhy8 through maxim or nebulosity ok, but i did feel at times that i was a little uncertain and went back and forth a bit to find good focus.. I'm working on a mask for the c11 and the refractors at the moment.

[Dennis]

Hi Mike

Some bright guy on CN has written an on-line Bahtinov Mask Generator (http://astrojargon.net/maskgen.aspx). Basically, you type in certain values for your ‘scope and then the application generates a file, with an extension of .svg (Scalable Vector Graphic) which most drawing applications can open.

However, I have tried this free on-line generator and the .svg file it produced gives me an IO error in my drawing application, Corel Draw.

Whether I have over protective Anti-Virus, Anti-Phishing and Firewall software and settings on my PC, I’m not sure. However, if you want to type in your parameters, generate the .svg file and e-mail me a copy, I’ll try to open in and if that succeeds (unlikely!) I can convert it into a PSD, TIF, BMP, etc.

Cheers

Dennis

[Dennis]

Hello,

Here is an image of Nunki using the Bahtinov Mask on the Mewlon 180 with the DBK21AF04.AS colour CCD. I had a TeleVue x3 Barlow plugged in. The attached file shows the separate R, G and B Channels as well as the original RGB image.

It looks like the mask is acting as a diffraction grating that splits the white light into its Blue, Green and Red components, presumably generating mono chromatic diffraction rings located at the points indicated by the respective coloured arcs.

Remember, whilst this particular mask was designed with the formula of Focal Length (=2160mm)/180, resulting in a Bar Width=6mm and Bar Gap=6mm, the image was captured at x3 the focal length, 6480mm.

Cheers

Dennis

[spearo]

Hi
I tried the link and it works well. Make sure you dont use decimal places in your specs
frank

[citivolus]

Well, I just tried this out visually on my Megrez 90 against my other masks, and have to say that it does definitely work as advertised. I was able to tell with precision where the critical focus zone was within at least 1/8 of a turn on my 10:1 Crayford focuser, even with my non-perfect eyes. It was so much better that, when the wind blew my old mask under the lawn mower, I didn't bother going after it :)

I do have one unresolved curiosity. I notice that a lot of people are using a circular mask to define the outside edge of their diffraction slits. The ends of these slits are not squared off, and I'm curious how much of an impact this will have on the diffraction pattern. Attached is the shape of the mask that I made.

Any comments?

Regards,
Eric

[TrevorW]

Thanks to Dennis's motivation and Houghy(David's) post of the mask for the 100ED which I re-scaled using the photocopier at work, then pasted the mask onto slightly thicker card, painstakingly with ruler and scaple cut out the bits. Now to make a frame for regidity

Call me tight but $82 for one is a bit steep and here is the finished product hopefully it works.

:thumbsup:

[allan gould]

I copied this jpeg and then in Irfanview rescaled it for printing out the appropriate size onto A4 paper. It gave me masks that worked perfectly on my ED80 and 127mm scope without any alterations except for the size.
What a really pleasant surprise and focus is absolute. I was never sure before where exact focus lay but this nails it perfectly. Just so easy to use.
Again thanks Denis and Mr Bahtinov.
Allan

[Dennis]

Hi Eric

The inside edge of the outer circular rim of my Bahtinov Mask is of a larger diameter than my mirror, so my mirror never “sees” this edge, although it does "see" it's own edge, i.e. the edge of the mirror.

Cheers

Dennis

[citivolus]

Thanks Dennis.

On further thought, the diffraction pattern is going to be spread along the slits width wise, not length wise, so the ends probably don't come into play anyway.

Ah, my wife is done with the camera, time to go steal it back!

Eric

[wunder]

Try saving it to your desktop and just double-clicking on it. On my computer, it opened in firefox and it looked like I could print it out. Of course, i don't need another one as I have about a dozen aluminum prototypes here at the moment. ;)

[montewilson]

Cool stuff - I will look further into it!

[Dennis]

Hi Wunder

Thanks for the tip, but I had already tried that and get the same “I/O read error” event though I can still open other .svg files either via “double clicking” or “drag and drop” or “File Open”.

I’ll check Corel Draw V12 to see if there are any SP’s or updates.

Cheers

Dennis

[h0ughy]

i used the online mask generator - this time i generated the svg files and then used the latest version of the GIMP to read the file and convert it from svg to photoshop - the results are attached. Corel 12 wont read the files even with a patch update Dennis:shrug:

[iceman]

12" dave? :)

[Dennis]

Hi Dave

Thanks for the feedback re the .svg files and CorelDraw 12. I already had CDr SP1 and all patches applied and it still gave the I/O error, although I can open .svg files from other sources.:shrug:

I downloaded and installed Inkscape (http://www.inkscape.org/)(free) and that opened the .svg files so I’m okay now.

Cheers

Dennis

PS - Inkscape is an Open Source vector graphics editor

[AlexN]

:) Dave, nice little list of masks.. wheres my C11?? Megrez 90, 102?

aww... I've been left out... **looks at his half finished mask on the work bench... Neeever mind.

[h0ughy]

give me the diameter of the scope, the focal length and the size of the centre blockage

[AlexN]

I was mucking about.. I've started making mine already.. :)

[TrevorW]

Houghy

I noticed the ED80 one has signifcantly smaller slits than the one I made up by rescaling your previous post for the 100ed, will this make much difference

Cheers

[h0ughy]

I just followed the formulae and the assumptions - it was either a 5 or 6mm spacing for a 80mm dia and a 600mm focal length
the online mask programme did the rest:shrug: i am yet to make and test - but i have printed it;)

[h0ughy]

well use it at your own risk:lol:

[TrevorW]

Based on the original formula (using my calcualtions) the slit width for an 80ED f/7 should be roughly 3.75mm. The original post of the 100ED mask from Houghy rescaled to 83% produces slit sizes of this width whereas the recent post produces signifcantly smaller slit widths which are not easily workable.

Cheers

[Dennis]

Hi Trevor

The “standard” formula is Focal Length/N(range150 to 200)=(Bar Width + Bar Gap). For shorter focal lengths we multiply the Focal Length by a factor of 3 giving:

3xFL/N=(Width+Gap)

An 80mm F7 has a FL of 560mm.

So, using the denominator at max value of 200, I get a 4.2mm Bar Width.
560x3=1680
1680/200=8.4
8.4/2=4.2mm

And, using the denominator at min value of 150, I get a 5.6mm Bar Width.
560x3=1680
1680/150=11.2
11.2/2=5.6mm

The x3 multiplier for focal lengths of say, less than 800mm means that you don’t have to cut the slits too thin.

Cheers

Dennis

[TrevorW]

Thanks Dennis what's 1/2mm between friends I thank you and Houghy for posting to/this thread hopefully it will help in solving my focusing woes.

Cheers

[Dennis]

Hi H0ughy

We’ve had a few days of cloudy nights (not CN!) so I’ve been playing around in Corel Draw and the layout of the Bahtinov Mask.

In my original design, for the Mewlon 180, I used a denominator of 180 to give me a Bar Widths and Bar Gaps of 6mm. I then laid out the horizontal bars/gaps and 20 degree bars/gaps using this same 6mm result. However, I noticed that due to the offset caused by the 20 degree angle, the angled bars gradually began to move "out of phase" with the horizontal bars the further I went above/below the middle horizontal bar.

So, I did some calcs in Corel Draw and came up with the following:

For my 6mm horizontal bars, to keep the bars in phase (i.e. connected) the angled bar widths and bar gaps need to be 5.6382mm.

I have attached a couple of files illustrating this. In terms of real world use this is probably not important, but if you are going to get these cut at a workshop, it might be worth making sure the master template is accurate? I’ve yet to print and cut the modified mask with the narrower bars/gaps on the angled side, but I’ll probably get around to doing this and testing it over the next few days.

I wonder if the mask generator allows for this offset?

Cheers

Dennis

PS - I know, I know; you’re probably thinking I have way to much time on my hands….:whistle::whistle:

[jerry3672]

No need to keep the angled side in phase. You only need to make sure that whatever angle you use is repeated in a mirror across the center axis. Remember a diffraction spike is created when the light wave is disrupted by a sharp edge. It will always be in the center of the light path when it is in focus. This is the root of how the Bahtinov masks works. The diffraction spikes on the horizontal side will shoot out 90 degrees from the cutouts, and always in the center of the light. the angled side will also shoot diffraction spikes out 90 degrees from the cutouts. So the mirror image design is important so as you get all of the light in focus the diffraction spike from the horizontal side will be in the center of the two angled sides. The number of slots and distance of the cutouts are only minor details.

[Dennis]

Hi Jerry

Thanks for the explanation, I suspected as much. Apart from a basic understanding of the central Airy disc and the accompanying 1st and 2nd order diffraction rings (which I can usually seen in-focus with the Mewlon 180 and my Vixen ED102 refractor at high magnifications), I’m still a little hazy on some of the wider aspects of diffraction, other than the more edges, the more I get!

Cheers

Dennis

[jerry3672]

Dennis- Good point! The more edges you have the more diffraction you get. However as our friends in Russia have found also too many slots will blur the spikes and make it a bit more difficult to focus. Pavels formula is designed to catch the peak of each light wave and stack them on top of each other to get the maximum diffraction spike that is clear. Farmer Dave posted his mask generator that works fabulously.

When I sat down and plotted a curve for different focal ratio scopes I saw a curve in the results. I drew a straight line down the middle and came up with a simple formula that works on every scope we tested. These were as small as an ED80mm up to a 16 inch Meade LX200. From F4 to F15. 26 different scopes so far. All of the work with the following math:

Aperture divided by 35 will give you the spacing for a center cutout and 8 up and 8 down. The other tip we did was when you get down less than 130mm we added one extra slot on the angled side only. this helped the spikes to show up a bit brighter.

[Dennis]

Hi Jerry

Thanks again for providing your insights and some experimental results from the research and development efforts of developing the mask for commercial release.

Not withstanding Pavel Bahtinov’s original talent, intuition and generosity in publishing his design, I think you have been very generous in providing guidance and helpful information on several forums that I have enjoyed visiting on this topic.:thumbsup:

I note that your posts have provided encouragement for others to better understand the principles involved, as well as providing helpful “nudges” to the army of DIY mask makers, helping them focus on the key design issues for optimising their own home made designs.

I hope that once the home made masks begin to un-stick, wilt and fall apart, their owners will visit your website and purchase the “last a lifetime” model! I feel certain I’ll be one of those after our next humid summer season down under!

Cheers

Dennis

[jerry3672]

Glad I can Help.

I'm a DIY guy myself. I enjoy reading posts and suggestions and passing on what I learn.

[spearo]

I'll be trying gout this mas next week
Knocked one of these over today for the C14,6 mm foam board (with black cardboard on both surfaces), might spray it with something to plastify it? but then that might cause some ondulations...not sure. Anyways, worth a try i guess
Overall diameter 16 inch inside circle diameter 14"

slits of 12mm (same width for closed/open/straight and angled sections)
will let you know how I go

The two small tabs will be bent, the large one is for handling it and has a hole in it to hang up the mask when not in use
cheers
frank
PS thanks to everyone and Mr Pavel Bahtinov for their contributions to the amateur astrophotography world!

[Terry B]

I have also made 2 of these things out of foam board, one for my 127mm and one for the VC200L.
They both work like a dream. I didn't bother painting mine and they are white. I can't see what difference it will make as it is only for focussing and doesn't matter if it gives a bit of dispersion etc.
Total price $16 for the sheet of foam board with a large amount left over for the kids to use on "craft"
:lol:

[Dennis]

Nice work Frank. Let us know how you get on using it.

Cheers

Dennis

[Dennis]

Hi Terry

I used a sheet of those square, interlocking foam mats that you buy in packs of 6 at camping shops, to use as a walkway at the entrance of tents.

A packet of 6 cost me $22.00 and the remaining 5 will be put to use for their original design intention – as tent entrance mats!

Cheers

Dennis

[Dennis]

Hello,

Here is an image of how my home made Bahtinov Mask performed when focusing on Io, one of the Galilean moons of Jupiter. You can just see the glow from Jupiter’s overexposed disc on the right edge of the frame. This really is a very, very neat and tidy way to focus when operating at such long focal lengths.

Celestron C9.25 with TeleVue x4 PowerMate.
DBK21AF04.AS, F40 at 9400mm efl.
Frames exposed for 0.5 secs.

Cheers

Dennis

[iceman]

Very cool, Dennis! I must make one of these for my 12"!

[TrevorW]

Dennis

This is the image I get from the mask I made but I'm still nto sure whether I'm getting as perfect a focus as needed.

Cheers

[Dennis]

Hi Trevor

I was playing around visually with my C9.25 mask last night and noted that depending on where I placed my eye in the exit pupil of the eyepiece, the centre “moving” line could move up or down ever so slightly within the “stationary” shallow cross.

With a CCD camera at prime focus or using the TeleVue x2.5 and x4 PowerMates I was able to nail focus despite the poor seeing.

Going back over some items that Jerry has mentioned, it is important to ensure that the mask is drawn and cut well, is mirrored around the centre, has clean edges, etc.

I’ve had heaps of fun and stimulation so far. I have enjoyed learning more about optics and diffraction during the drawing, fabrication and use of several prototype masks, but it is likely that eventually, I will purchase a rigid, professionally cut model. I consider it a small investment for a life times focusing.

Cheers

Dennis

[AlexN]

my home cut one made from foam core board was a bit of a miss. My patience ran a bit thin towards the end of the 11 inch mask and the cutting got very average. I bit the bullet and bought one for my c11 and the refractors.

[Dennis]

Here are some examples of focusing with a single mask on the C9.25 F10, native focal length of 2350m.

At prime focus (2350mm).
With TeleVue x2.5 PowerMate (5875mm).
With TeleVue x4 PowerMate (9400mm).

The Bahtinov Mask was printed on two A4 overhead transparencies and then the gaps were cut out using a Stanley knife and steel rule, before gluing the mask to a camping mat foam ring.

Cheers

Dennis

[TrevorW]

Forgot to mention my image of the mask was focused on Venus, so as you know fairly low on the horizon and obviously some atmospherics came into play. Also haven't tried it for visual focus yet

Cheers

[Dennis]

I was wondering how you managed to obtain such a bright diffraction pattern – now I know why!:lol:

It’s best not to focus on an extended object such as the disc of a planet; the mask really requires a point source of light to be at its most effective. However, I do focus on the Galilean moons of Jupiter if I can’t find a nearby bright star. I’ve found that their discs are so small that they approximate to a point source for the range of focal lengths I’ve used so far.

Cheers

Dennis

[Dennis]

I forgot to add that an extended disc can be considered to be a collection of point sources dotted around the circumference of the disc, so the mask can be fooled by these multiple point sources.

Cheers

Dennis

[AlexN]

Read: do not attempt to focus on the trapezium in Orion... :)

I tested mine out focusing on Canopus then taking a 45 second sub of Canopus and it was still just that little bit soft... but some of my scalpel cuts were a bit haggard... Thank god I'm not a surgeon.

[Dennis]

Hi Alex

A 45 sec exposure, that surprises me - what was your optical system set up?

With the mask on either the C9.25 or Mewlon 180, with the TeleVue x4 PowerMate plugged in, I was focusing the DBK21AF04 using exposures of between ½ and 2 seconds on mag 1 stars?

Cheers

Dennis

[Garyh]

I might try making one of these up?
How do they work on fast f/ scopes? Also where can you buy foam board?
cheers Gary

[TrevorW]

Foam board can be purchased at art stores or someone like Jacksons Drawing supplies. They work just as well on fast scopes mines an 80 f/7 and my others a 120 f/5.

Cheers :thumbsup:

[Dennis]

Hi Gary

I’ve not yet tried foam board, but what I can say, is that the overhead transparency material seems to make a very good mask.

H0ughy has posted some standard sizes and if you can print one of these on A4 overhead transparency, then the slots are reasonably easy to cut out with a sharp Stanley knife and steel rule. The transparency material is surprisingly tough and doesn’t seem to tear easily. I always cut with the steel rule lying on the black bar, leaving the clear slot visible – it seems easier to see the edge that way. I then cut all the slots along one long edge and turn the work around and do the other long edge. Finally, I cut the narrow edge at the middle bar and at the inner circumference.

I bonded the mask (once cut out) onto a closed cell foam ring made from those camping tent door mats that interlock together. I used contact adhesive.

Here are examples of my C9.25 mask and Mewlon 180 mask. The C9.25 has a central aluminium bar to help with stiffness and a nylon bolt to allow me to place the mask on and remove it with the dew shield fitted. When fitted, the C9.25 mask sits on the metal frame surrounding the corrector plate. The Mewlon mask just sits over the end of the OTA. The last file is a printed but uncut A4 transparency.

Cheers

Dennis

EDIT:
The C9.25 mask required 2 sheets of A4 transparency paper. I printed the central bar + left hand horizontal bars on 1 sheet, then the central bar + right hand angled bars on the 2nd sheet. After cutting out the slots, I overlapped the central bar and glued them together.

[Garyh]

Thanks Trevor and Dennis, I downloaded the 12" GSO that Houghy uploaded (thanks Houghy) and shall rescale it to suit the 8" scope...
cheers

[AlexN]

Dennis,

optical system was Megrez 102 - UV/IR filter - QHY8, what I meant by that was, I used Nebulosity 2's focus routine with the mask to get the focus to where the mask said it was perfect, removed the mask, then took 5x45 second subs.. Its a little mini-project Im doing when the neighbors turn their flood lights on, or when the moon ruins clear nights... Photographing the named stars... Anyway, after focusing with the mask using the focus routine then removing the mask and taking the images, I found it was still a little soft..

[leon]

Maybe I will be blasted out of the water for this comment, but, so be it.

I can't really see why these mask things are such the rage all of a sudden, :shrug: and not to sound like a smart arse, I have been doing diffraction spike focusing every since I have used the Tak.

With the Tak you can look through the angle finder attached to the Canon 5D, slowly move the focuser in and when the star light diffracts into different colours, you know your on the money. ;) surly this applies to most scopes.

You can actually see it snap into focus, never misses.

I have never used any other form of focusing other than this method, but than that is just me, I suppose, and I'm half blind to boot.

Leon :thumbsup:

[AlexN]

Leon, Unfortunately the QHY8 does not have a view finder... :)

[leon]

Ok Alex point taken, have to admit I didn't consider any other imaging devices other than DSLR's, sorry, my oversight, :ashamed: maybe I have been a little harsh in my comments. :whistle:

Leon :thumbsup:

[Dennis]

Hi Leon

No one should be blasted out of the water for describing their experiences, techniques, approach or set up. There is always something to learn from each other and without the healthy exchange of information here on IIS, I’m sure we’d all be the poorer in our hobby.!

A scenario where the Bahtinov Mask really shines, for me, is in long focal length imaging. Trying to eyeball best focus at focal lengths of over 3000mm can be time consuming and often leaves me with a niggling doubt.

I find that the Bahtinov Mask helps me converge to best focus rapidly and confidently. But, there was life before the mask and I’m certain there is life without it too.:lol:

Cheers

Dennis

[Dennis]

Hi Alex

Ta for the clarification.

When I’m using the SBIG ST7, I converge to focus quickly with the Bahtinov Mask and then confirm best focus using the Focus routine in CCDSoft, the SBIG camera control software. This gives me the best of both worlds, vis-à-vis:

Rapid convergence to a really good focus using my eyeball.
Touching up final focus (if necessary) assisted by the software algorithm!

Cheers

Dennis

[Ian Robinson]

Unclear on what they do ?

[h0ughy]

well either read the entire thread or go to the cloudy nights link on this:shrug:

[wunder]

The diffraction pattern on the masks is much brighter than some other methods of adding diffraction spikes. You can use much dimmer stars. Also, the way the center spike is framed in the outer spikes makes it very easy to get it just right.

[spearo]

Folks,
let me tell you...I've just had a go with my home-made Bahtinov mask this weekend.
All I can say is:
WOW !!!!
images being processed right now i'll be posting soon.

The mask is simply wonderful. I got focus either at F/11 or with the 6.3 reducer on the C14 without any problem (i do use the live view function of the Canon 20Da which really magnifies the image)

I got great focus in seconds and it was so easy, every time i'd line up on a star to synchronize and refine the next goto to a target, I'd put the mask on and check the focus.

make one for yourselves, you wont regret it.

frank

[RB]

Thanks for posting Frank, I want to make one for my 20Da too.

:thumbsup:

[h0ughy]

Which One RB? LOL

I found that foam core board is not the best - bad idea for the ed80 and the 127ED - yet to do it for the c8 and the 10". laser cut is the answer

[RB]

:lol: :P

[ozstockman]

Hi Jerry,

If you want to contact Pavel I am sure that you can find his current email on http://www.astronomy.ru/forum/index.php?action=profile;u=408

I have never met him but I don't think there is more than one Pavel Bahtinov(Павел Бахтинов), who lives in Samara in Russia and is keen on astrophotography. Moreover the page above has a link to astrophoto.chat.ru as a personal webpage.

I reckon that Pavel can speak English or at least can write and read in English but if you have problems or need any help communicating with him let me know. I speak Russian and can help with it.

cheers,

Mike

[wunder]

Hi Mike- thanks for your offer!

We were able contact Pavel and are getting the details worked out. Thanks for your offer on translation- we may take you up on that, as Pavel doesn't speak any English.

[wunder]

By the way- our original problem contacting him was his Spam filter software intercepted all of our emails! :)

[h0ughy]

Well i posted some infor last night - then deleted it this morning. for those first 5 templates it was going to cost 140.80, i am getting a new quote for a further 3 templates for Mr Simmons. these will be made from 1.2mm 304 grade stainless steel. I make a fundamental stuffup in the design - now corrected (thanks Dennis).

[Dennis]

Hi Dave

Thanks for generating the drawings for the C9.25, M180 and Vx102 – I appreciate your work!:thumbsup:

It’s a real pity that you couldn’t get the Gold Leaf Crinkle finish, with a contrasting Platinum Inlay and Diamond Encrusted Edges! Oh well, stainless steel will have to do!:whistle:;):lol:

Cheers

Dennis

[h0ughy]

you know between my first submission and speaking with you they cut out the entire first lot this morning......after i told them to hold.....:whistle:

their mistake (for cutting) my mistake for not ordering the gold leaf and diamond encrusted coatings:P the platinum inlay was tacky

[Dennis]

Just make sure that only the non-diffracting edges are decorated with the Diamond Encrusting! :lol::lol:

Cheers

Dennis

[h0ughy]

:D
OK the quote to have 8 variable sized masks cut out of 304 grade Stainless steel will cost $192.50 making them on average $24.06 each for this batch. Obviously the larger ones would be dearer normally. I think this is excellent value for money - and an economical way of getting a decent focus mask. Many thanks to Dennis for his help and the input of everyone who posted advice. and see post 197 for the CAD dxf file :)

I have ordered them and will have them next week - enough time to spray one side matt black and add felt to it before IISAC.

[iceman]

In your order, can you include one for my 12"? And maybe one for my ED80 too?
Do you need the OD of my tube?

[h0ughy]

My scopes and Dennis's focus masks are already ordered - so that job is currently being done. I see no reason why a seperate quote cannot be done - just need to draw up the plate mask - need the tube diameter so that it can sit in/on the tube and the centre obstruction diameter. Can submit the order to C and J Sheetmetal, I have included the quote that I got for reference.

[h0ughy]

Ok I picked up my masks today, and Dennis's. they look rough but very promising :thumbsup: they are not flimsy, are tough - i dropped my ed80 a few times to see how it would go - no worries :whistle: the bigger ones might deflect wit ha lot of force behind it - but will not break, are not flimsy, and hold their shape rather well. I need to get some felt now – and flat black paint. Looking good!!!!

For those who made inquiries they charge minimum $35 dollars each for the ED80 and 127ED size for those as they are one off’s and require to be set up. I bought mine in a job lot – hence the cheaper price. I have provided the CAD file of what I have and the contact details for the company, and how much they are minimum for single one off’s. it is up to you what you do – I will not be acting as a go between.:shrug:

[Dennis]

Hey Dave

They look great – thanks heaps for including my masks with your order, I really appreciate your efforts in drawing them up and placing the order. Just let me know how much and I’ll transfer the funds.:thumbsup:

Cheers

Dennis

PS - Thanks for the piccies, I'm drooling!

[allan gould]

Did a small experiment last night after spending some time cutting out a Bahtinov mask from a template. I just printed out Davids template onto a piece of very clear and uniform acetate sheet used for making overhead slides/presentations and stuck it onto the front of my scope theorising that you are just looking for a diffraction pattern and not resolution.
Worked perfectly, thus no need for tedious cutting out. Might be worthwhile for those of use not allowed to play with sharp instruments even with supervision

[Dennis]

Woohoo, my x3 Bahtinov Masks arrived today, care of Mr H0ughy and Australia Post. Now all I need to do is prime them, spray them with flat black and make the closed cell foam fittings for the various OTA’s.

Needless to say, the weather forecast is cloud and rain for the next few days….

Thanks heaps Dave – the masks look really well made and if I had two of the smaller ones, I would even consider getting my ears pierced so I could wear them as a set of uniquely designed, ornate, ethnic looking earrings!:lol::lol:

Cheers

Dennis

[Dennis]

Hi Allan

Arguably, if the plastic film is thin, taught and uniform, it should not affect focus when placed at the objective end of the ‘scope as the light rays are parallel there?

It’s only at the eyepiece end where the rays are converging that you will affect focus by placing e.g. glass filters in the optical train.

I did use an uncut, inkjet printed overhead transparency for my 1st prototype and it did produce quite a messy and horrible looking diffraction pattern, even with the A4 film glued to a foam collar placed over the end of the OTA.

When I’ve finished assembling my stainless steel masks, I’ll run some tests to compare the results with a non cut-out transparency, a cut-out transparency and the S/S masks just to see the differences, if any.

Cheers

Dennis

[h0ughy]

Hip suave and sophisticated – Big D you are perhaps focussed in the wrong direction….:rofl: is the fact you got some new gear today the reason you have storms and rain at the moment?

[AlexN]

Dennis... This weather is your fault!???

Grumble Grumble.

[Quark]

Hi Dennis,
Only recently did I discover this amazing thread.
After working my way through the whole thread and upon completion and testing of my first prototype of the Bahtinov Mask I felt compelled to add my comments to your thread.

Firstly thanks so much for introducing the topic to IIS.
What a pioneer this Pavel Bahtinov is and just how significant is his contribution to amateur astronomy.

In my humble opinion this is one of the greatest innovations that I have seen in amateur astronomy and rates right up there with the Telrad, Dob and Surrier truss. Surely there must be some sort of design award that this must qualify for.

My prototype is for my 16" F4.5 Eq mounted Newt. I used light cardboard, with the outer rim reinforced with a heavier cardboard ring. I applied the formula to my specific scope, spent hours very accurately marking it out then cut it out using a machined steel rule and a very sharp blade.

I designed and built my own secondary mount and it utilizes a central piece of studding that protrudes out past the end of the telescope tube. I was able to punch a matching central hole in the mask and will reinforce that area of the mask. The mask slips nicely over that studding which locates the mask quite well.

Obviously I have read the thread and taken note of the results that you all have been getting but there is nothing like experiencing it for your self. I have a JMI motofucus on my scope and spent about an hour running it in and out of focus, watching the central line moving up and down is a special thing, even dragged my better half and rug rat down to my observatory to check it out.

Could not believe just how crisp the lines of the diffraction pattern appeared.

All in all a most memorable experience and will be a boon for astrophotography.

Pavel Bahtiov take a bow and thanks again Dennis for making all on IIS aware of this stunning advance for amateur astronomy.

Regards
Trevor

[Dennis]

Hi Trevor

I find it quite inspirational that even in this era dominated by sophisticated, expensive, high-tech equipment, such a relatively simple and passive focusing aid like the Bahtinov Mask, has had such an impact on our hobby.

Pavel Bahtinov’s remarkable contribution to achieving perfect focus, with such accuracy and ease, through the use of such a simple tool, will surely go down in history as one of the most significant advances in amateur astronomy - a brilliant aid to focusing.

Cheers

Dennis

[Octane]

Dennis (and/or anyone else),

I have skimmed this thread -- looked at all the images attached.

I was wondering, if I was to be attempting focus with an ED80 with a William Optics 0.8x field flattener/focal reducer, will I have to buy/make two versions -- one for the native focal length, and one for the reduced focal length -- or will just one suffice for both focal lengths?

Apologies if this has already been definitively answered.

Regards,
Humayun

[AlexN]

Humayun, One will suffice... I use the same mask on my C11 at both F/10 and F/6.3

Also, I have one for my Megrez 102 that works with and without the WO 0.8x flattener/reducer.

[Octane]

Alex,

I thought that may be the case.

Thanks, heaps!

Regards,
Humayun

[Dennis]

My experience so far has been that my prototype printed mask on A4 transparency (with the gaps cut out with a sharp knife) for my C9.5 has worked at the native focal length of F10, with the Celestron Reducer/Corrector at F6.3 and also using various PowerMates from x2.5 to x4. A similar story for the Mewlon 180 at F12 (native), F9.6 (Reducer/Flattener) and all the way up to F48 with a x4 PowerMate.

For DMK webcams and my Canon 40D, I simply eyeball the focus as the camera control applications I use don’t have a means of measuring focus by e.g. FWHM. However, when using my SBIG ST7E and CCDSoft, I may end up making very small tweaks using a motorised focuser, as CCDSoft (camera control software) has software assisted focusing.

Cheers

Dennis

[Octane]

Dennis,

Thank you so much for that, much appreciate! :)

Regards,
Humayun

[h0ughy]

well i painted up my masks today and places clear sticky rubberised feet on them and a stick on handle. will post pictures later as every camera I have has a flat battery at the moment

[RB]

Clear sticky rubberised feet, now what a great idea, how did you think of that? :P :whistle:

[Octane]

David,

You didn't order any extra 80ED sized masks, did you? :P

If we mention your name to the laser cutting mob, do we get a 90% discount? ;)

If I send them the 80ED file that you've created, they'll know what to do with it, aye?

Regards,
Humayun

[h0ughy]

as i said earlier i wont be handling peoples individual orders. you are most welcome to drop my name - might not mean much though. the file is all you need with it noted you only want the ed80 and the 127ed. remember to as for shipping to yourself:thumbsup:

[Dennis]

Hi Dave

I finished my masks over the weekend too. I sprayed each one with a metallic primer followed by a coat of flat black. The C9.25 and Vixen have a closed cell foam collar fitted underneath, so that they sit on the C9.25 metal corrector plate cell (actually on top of the 6 screws on the cell) and the Vixen objective cell. Both can be fitted even when the dew shield is fitted on the OTA.

The Mewlon Mask just fits around the end of the OTA, held in place by a closed cell foam collar glued to the S/S mask – I used a contact adhesive for all cells and left them to outgas over the weekend just to get rid of any solvents. For the C9.25 and Vixen 102, I added a knob which allows me to grip the mask and lower it into the dew shield.

The larger masks are very rigid so good choice on the 1.2mm (1.5mm?) S/S.:thumbsup:

All I need now are some clear skies to try them out!

Cheers

Dennis

[h0ughy]

they look fantastic Dennis, I like the knobs - very sauve!!! I sprayed mine with flat black metal etching/primer top coat in the one go. i used SS hooks as the handles - you will see them this evening when I take a picture of them. :thumbsup:

one thing I thought the mewlon was to fit on the inside and not like what was in the image you posted? Did i stuff up?:shrug:

[Dennis]

Hi Dave

No – you didn’t stuff up; in fact, you delivered on time, to budget, to specification and met the customer requirements fully. You also provided an added value service by generating the drawings for my set of masks as well as arranging all the procurement and shipping, so top marks to you my son!:thumbsup::thumbsup:

The Mewlon mask was always intended to fit on the outside of the OTA, as unlike the C9.25 and Vixen 102, the Mewlon does not have a front end cell for the mask to rest on.

I look forward to seeing your photos!

Cheers

Dennis

[h0ughy]

here are my masks Dennis - apart from the paper bag that i use everyday:whistle:;)

[netwolf]

Great job guys looks great, and is it just me or do they look like Pacman. I wonder if that's how the inventor got the idea, maybe he likes the Pacman game.

[Dennis]

Hi Dave

Nice work in finishing them off; what a lovely collection of masks….and, what a lovely collection of optical tubes you must have…LOL!

My Vixen 102 mask will also double up for the Canon 400mm F5.6 lens; I only wish the weather would clear up so I can fit the masks and test them.

Cheers

Dennis

[peter_4059]

Hi Dennis,

I've been watching this thread with interest for a while and thinking this mask was a lot of mucking around to make but finally decided to give it a try.

I cut one out of some thick plastic last night - tried it out tonight and works better than I expected. The point of best focus was much easier to find than with the Hartmann mask.

I'll have to make one for the 10" Newt now.

Thanks for the tip.

Peter

[AlexN]

Ya done good Peter... looks very sharp!

[Dennis]

Nice work Peter. The middle star looks the sharpest to me – is the sequence before focus – at focus – through focus or is it just my eyes!

Last night I noticed than when using the mask visually with my C9.25 and Pentax XW14 eyepiece, I could cause the central bar to “move” when I moved my eye around the eyepiece, so it does seem a very sensitive method of focusing.

Cheers

Dennis

[peter_4059]

I should have included a bit of explanation and the corresponding images either side of focus with the new design. The last three images are my attempt at either side and in focus with the Hartmann mask. I was using the same step size based on the focuspal motor focus controller, repeating the routine with each mask. I found it a lot more difficult to pick the sweet spot with the Hartmann mask as on the live screen the images look very similar for a few steps around the point of best focus. It is easier to pick the difference from these images when you see them side by side.

Peter

[redsquash]

HI David ,
I am responding to your PM on the forum , as other members may have a similar question.
How effective is this mask for fast minor , grab and go scopes ?
Are the effects similar for visual observations or is this predominately an exercise for astrophotogphy?

My newton specs are
130mm aperture
f/5
focal length 650mm
45mm or 35% central obstruction by secondary mirror

I tried doing the the calculations but there was a term or two which threw me so I just guessed the data.
Can you help with the claculation.
given the limitation of this scope is there going to be a noticeable difference

[sheeny]

I finally got a chance to test my aluminium framed fishing line version...

:thumbsup::thumbsup::thumbsup::thum bsup::thumbsup::thumbsup:

It works great!:D

No messing with changing exposure settings on the camera either.

I did notice that when the image was out of focus there are a mess of diffraction spikes... maybe that's something to do with some fitting errors or the fact that I mis-strung my mask (so the angled lines are not in the same plane as the horizontals).

One thing I did notice though, is that the diffraction spike don't work the way I expected... out off focus the intersection moves along the horizontal spike and doesn't coincide with the star image... this seems to be different to what the others are doing. I think it is because I changed the direction of the strings to make the mask more structurally sound... I didn't think it would change the way it works like that.:shrug::screwy:

Anyway... it works a treat!

Al.

[h0ughy]

i will have a look when i get to work on Monday - have left the file on m PC there:sadeyes:

[Dennis]

Hello,

The guy (David Polivka) who designed and coded the “Bahtinov Mask Generator (http://astrojargon.net/MaskGenerator.aspx)” has just released V 0.4. This (free) Generator is a web based application that generates a “Bahtinov Mask” pattern suitable for your OTA based on your input parameters.

From Dave's website:

Changes:

Version 0.4. 2008/10/18
Moved to new hosting provider.
Complete rewrite of mask generation code.
SVG opens fine in Corel Draw 12 (and, I think Corel Draw X4) now.
SVG now converts to DXF cleanly in Inkscape. You should be able to take the resulting .DXF directly to a machine shop/laser cutter/etc.
Split the existing generator page up into 4 pages (Overview, Generator, FAQ, and Change History) to make them more manageable.
Segregated parameter entry so it's not so overwhelming - broke it into Basic parameters, Advanced parameters, and Print options.
Entry validation added.
Appropriate parameters will accept numbers with decimal points.
Added an option to make the center of the left-hand side a bar instead of a slot.
Added an override for the slot width.
Added an option to scale the SVG to 72DPI for use with Adobe Illustrator.
Cleaned up the text a bit - now consistently use "slot" to mean an opening in the mask and "bar" to mean a solid bit.

Thanks once more to Pavel for his original design and to David for his superb implementation of the Generator and his on-going development and support. Here is the link to astrojargon (http://astrojargon.net/default.aspx), where the Generator (http://astrojargon.net/MaskGen.aspx)is hosted.

Cheers

Dennis

PS – For Canon 40D owners Dave has also written an application (http://astrojargon.net/40DShutterCount.aspx)that can interrogate the 40D and return the value of the number of shutter operations.

[I.C.D]

Dennis,
What program do I need to run the Generator
Ian C

[Dennis]

Hi Ian

The “Generator” is a web based application where, for the simple version, you enter the details of your telescope aperture and focal length, followed by how thick you want the outer ring to be.

The “Generator” will then produce a “bahtinov.svg” file which you then save to your PC. “svg” stands for Scalable Vector Graphics (SVG) which is an XML specification and file format for describing two-dimensional vector graphics.

A program called Inkscape (http://www.inkscape.org/)(a free and open source vector graphics editor application) can open .svg files and I know that CorelDraw12 now opens the .svg files from the “Generator”.

Cheers

Dennis

[Omaroo]

Sorry to come in on this so late!

I ran the generator and made up a template for my FS102 (820mm f/l, 102mm clear aperture and 150mm overall diameter). I then transferred it to a piece of thick-ish plastic card I have (left over from dewshields) and marked it out dot by dot. After cutting two slots and suffering stuffed knuckles with the pressure on the knife, I stopped. It's a crazy way to do it.

Question - is anyone making these at all now? Silly trying to get one from someone before Lostock - so I guess I'll attempt to borrow one up from some poor sap there (I'm coming for you h0ughy!!) and see how they work.

Anyways, just checking to see that someone either is or isn't producing any more....

[gmbfilter]

Try this http://www.iceinspace.com.au/forum/showthread.php?t=35971
I got on posted next day. Made from some sort of black plastic
Works a treat

[h0ughy]

contact the firm that did mine - you can open the file you produced and create a dxf file and send it to them - with express post maybe you can have it before you go:shrug:

[AlexN]

Had to post in here... I used my mask on the M102 tonight... first time I've used it on the refractor... Its just.... Bliss.... normally I'd spend at least 25 minutes getting it just right, and even then I'd be questioning it from time to time... Not tonight...

I never thought I'd say it... focusing is one of lifes simple pleasures !

[jerry3672]

You can find Bahtinov masks at www.spike-a.com (http://www.spike-a.com)

[I.C.D]

David,
Did you say you have some of these mask made if you did how do i go in getting made .I have a Meade lxd75 200mm sn can you help or point me in the right direction

Ian C

[h0ughy]

Ian read the entire thread and look at the attachments - if you cant figure it out by then then I will draw something up for you - but you then have to get it made (see previous posts)

[Omaroo]

OK - a bit behind the times, but I received mine yesterday arvo in the mail. Trying it briefly last night on a couple of bright stars I suddenly see everyones' point - that's the quickest perfect focus I've ever managed. Three seconds and it was all done. Thank you Pavel. Brilliantly-simple ideas work the best.:thumbsup:

[Dennis]

Yep – here’s a single raw frame from the Canon 40D showing the Bahtinov Mask hard at work, in full, glorious technicolour! The example is a full sized crop from the centre of the frame, no processing performed.

Cheers

Dennis

[netwolf]

This idea of using different holes to use one mask for multiple small refractor scopes is quiet a neat idea. So you can use the same mask from 80mm to 110mm
http://www.focus-mask.com/MORE_INFO.html

[Tandum]

Mine only works for me on bright stars. It's very hard to see the bands on anything but the brightest of stars. However, I only use it with live view on the 40D at the rear end of VC200L.

[Dennis]

Hi Fahim

I saw this website on CN in the DSLR Group, where it generated a bit of comment as the site originally didn’t appear to acknowledge Pavel Bahtinov as the originator of the mask. I think the site was then soon updated to mention his name.

Cheers

Dennis

[AlexN]

Rather than just looking on live view, try taking a 3 sec exposure (obviously with tracking on) I found the same thing with my CCD, doing the short 1sec preview exposure on dimmer stars it was near impossible to make out the diffraction spikes, however with a 3sec exposure, then inspecting the image at full resolution, it is rather easy to see even on dimmer (mag 4~5) stars..