Polar align a Dob to track more easily ?

[paulF]

Hey guys,
might be a silly question but just wondering if polar aligning a DOB would mean that i will only have to use the alt bearings(move it a up and down only) to keep my celestial object in sight ?
If so,how can one polar align a dob mount?

Cheers

[erick]

Paul, you have to point the base towards the south celestial pole. An Equatorial Table achieves this for dobs. Google that term or search for it on this site.

You still have to move in "alt" and "azi" directions, but they become Dec and RA directions.

[paulF]

Thanks for the prompt reply Eric :thumbsup:
I found the below thread which seems to be very informative!
http://www.iceinspace.com.au/forum/showthread.php?t=27173&highlight=Equatorial+Table

Will come back with some thoughts after absorbing all the info from the thread :)

Cheers again

[paulF]

Hi Eric,
here's my understanding of it after all this reading :)
If i point my dob base to True south and put it on a wedge on a 56degree angle (i'm in sydney), that would mean that the mount is polar aligned and i only need to move the scope up or down to follow one specific celestial object ?

Is the above reasoning correct please :) ?

Cheers

[barx1963]

If you tip your dob base up and point the axis of the base due south, then the azimuth axis become the RA axis. So simply turning it with the azimuth will enable it to track.

Malcolm

[paulF]

Thanks Malcom :)
What do you mean please when you say axis of the base ? It's circular so not sure what to point South!

Cheers

[barx1963]

The axis in this case is the bolt in the middle of the "lazy susan". This needs to point at the South Celestial Pole.
May I ask why you are wanting to do this?

[paulF]

Apologies for being a bit thick here but i still don't get it!
I can see the bolt in the middle of the Lazy Susan but don't understand how i can point it North as the base is circular!!!
I'm trying to build a slow motion manual drive and thought that if i only need to control one axis(east-west), i can do some manual tracking for Planets for astrophotography to get longer avi's.

Cheers

[Blue Skies]

Google "Horseshoe mount" or "Horseshoe equatorial" and see what you get. I think it will match what you're aiming for.

To achieve what you're after one of the axes of rotation has to be parallel to the axis of the earth - the basis of an equatorial mount. You need to change the mounting so that the axis represented by the bolt is tipped to match the angle of the axis of the earth at your location - or be pointing towards the south celestial pole (as they are the same thing really).

[paulF]

Thanks for that Jacquie.
I am trying to achieve the below with my existing dob mount.
It seems like simply tilting the whole base by sitting it on a wedge to be parallel to the axis of the earth will do the job!Not a clean job by the looks of things but doable i believe.

Malcom, now i understand what you meant about the bolt :)

Cheers

[erick]

UPDATE - now, of course, I was wrong in some things I said :ashamed: And you were all too polite to point it out! ;)

An equatorial table won't convert your Alt and Azi movements into Dec and RA - Duuuh! It is really designed for tracking purposes - keeping the object within the field of view. And it does so with one motor drive - of the table. I'm fairly sure this is good for visual, but not for photography since, if I understand correctly, you will get varying amounts of field rotation depending where you point to in the sky.

Yes, if you can tip the base up so the bolt through the base faces the South Celestial Pole, then you achieve the equivalent of an EQ mount. Add a motor drive in what was the azimuth direction and you will have tracking, and without field rotation (think I'm right on that last statement).

But tip your dob up to the required angle in Australia, and I reckon it will soon fall out of the alt bearings :sadeyes: I think you'd have to be a lot further south to make it practical.

[jenchris]

You're right about the field rotation. If you're polar aligned you eliminate it

[Barrykgerdes]

Hi

This is a question that often comes up from experienced users as well as beginners.

It is possible to tilt the rockerbox assembly in some Dob mounts so that the azimuth (base) rotates about the polar axis allowing the Alt rotation to become the declination and so allow tracking an object by just moving the scope on the RA (old Azm) bearing. However mechanically it is usually quite impractical without major rebuilding to do this.

A second way that many call an equatorial mount is to obtain an equatorial platform, mentioned earlier in this thread. However this is not a true equatorial mount and only works over a limited range by a tilting arrangement that allows a single motor to control the tracking in the RA direction. My last check on the price showed them to cost as much as a S/H EQ6 that will support a short 8" Newtonian.

The advantage of the dob mount is its simplicity of mounting a large aperture Newtonian telescope for a relatively wide field of view particularly when viewing DSO's.

If tracking is your main requirment an 8" Newtonian on a GEM will cost you less in the long run and you can save your dob for other purposes or sell it.
　
Barry

[mswhin63]

So far a equatorial platform has a reasonable cost if it is done in a DIY fashion. My first equatorial platform cost approximately $150 to build. That is quite reasonable and as you have seen from my Astro photography images they produce quite a reasonable job. Unfortunately they have not been up to produce a good long-term tracking for deep sky, mostly because the motor ratios were not low enough and I was getting a lot of stalling.

My new design is a lot more robust, smaller and incorporates aluminium radii and I plan to install stepper motors.

I hope one day that I will be able to sell these as kit so that people can put them together. This unfortunately will take a little bit of time as I am still wearing a sling from my shoulder surgery and I cannot progress any further than I have at the moment. I think if it is done right and effectively the cost could be right down.

[paulF]

Thanks for all the replies guys :)
At least now i understand how the whole tracking thing works :)
I only want to track planets so will try fiddling around with my existing mount and see if i can come up with something. Saving my big bucks though for a GEM and a refractor.
Cheers and thanks again :)

[gary]

Hi Paul,

Just by way of background, these days, for larger Dobs (e.g. 16" and up) that have
tracking, most users use motors on both axes and track simultaneously both in Az/Alt under
control of a computerized controller.

This is also exactly what most of the world's large professional 8m and 10m class
scopes do. They are all built Alt/Az but use a third motor to counter field rotation
for imaging. Modern computerized control makes this possible and practical and
large Alt/Az mounts are easier to engineer and construct than equatorial mounts.

Back in the 1970's, when 4m class scopes such as the AAT at Sidings Springs
were built, they were usually mounted equatorially. However, the AAT was the
first large telescope to rely entirely on computer control and its success, in part,
meant that computer control for all large telescopes built after it would be the go
and that therefore Alt/Az configurations would be practical as well. So the era
of large equatorial scopes pretty much came to an end.

Similarly, before computer control of amateur scopes, equatorial platforms were
popular. They can still be a good choice for smaller scopes (e.g. 12" and below)
and though less common now, there are still a few out there.

As one builder of large commercial Dobs once told me, he never intended
for his scopes to be mounted on an equatorial table. The scopes were designed to
be sitting on the ground and all the careful engineering calculations of the forces
and stresses involved go "out the window" once they are tilted on their sides.
Having said that, many users mounted large Dobs on equatorial platforms and for
example there is a 25" Obsession at the ASNSW site at Wiruna that is configured
in exactly this way.





Hi Eric! No, you were right the first time. :) Mounting your Alt/Az mount atop
an equatorial table will convert it to a true equatorial platform. Field rotation is then also minimized.



Hi Jennifer!

Very strictly speaking, you minimize it. You never quite eliminate it.
This is true of all equatorial mounts, not just Dobs on platforms.
Because of the effects of refraction, unless an equatorial mount
is dynamically adjusted in altitude (elevation) as well there
is still some residual field rotation. For practical purposes, since imaging times
are typically short and the fields of view typically narrow, on most amateur
and professional equatorial mounts, the amount of residual field rotation is
so insignificant as to not be a serious problem for imaging. Some scopes, such as the
U.K. Schmidt though, which has a very wide FOV, can also dynamically adjust
in altitude as well whilst they track in RA.

Anyway Paul, I hope you may have found at least something in the above interesting.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[erick]

OK, I'll keep "shooting from the hip" all the time then :D

But I cannot visualise it, Gary. Isn't it the curved surface of the EQ table that moves in RA? How does the Azi movement of the dob become an RA movement? If I move the scope in Azi on a table, the OTA pointing direction surely doesn't move in the RA direction? :confused2:

See attached, thanks to:- http://www.backyardvoyager.com/EQplatform.html

Perhaps we'd better go to another thread - "Equatorial tables for Dummies"? :P

[gary]

Hi Eric,

You're faster on the draw than me. :) I was still editing my original post when you
provided your follow-up and I was in the process of correcting where I typed that
Az becomes RA and Alt becomes Dec. The key to appreciating what happens
when a Alt/Az mount is placed on a platform is that a new third axis is introduced which
is a polar axis. If I don't spot one online, I will try and create a graphic to help
demonstrate it, as it is not obvious to many upon initial inspection the first time.

The good news though is that when a Dob is mounted on an equatorial platform,
the third polar axis transforms it into an equatorial mount. Once the platform is
polar aligned and assuming the platform is tracking, any arbitrary point in the
sky to which the scope points will then remain within the FOV.

Best Regards

Gary