Focus issues with Nexstar 8SE...

[truss]

... and it's not what you think.

My 8SE is visually tack sharp with my assortment of eyepieces, however I can never achieve decent focus when I've got the camera hanging off its arse. What "appears" to be in focus is quite soft, I'm not sure if it's a back focus issue or a collimation issue (usual collimation tests seems quite good).

Any ideas?

[Outcast]

First of all, I assume you are not running out of travel on your focuser?

Are you trying to focus the object by eye or are you using a Bhatinov mask?

If you are looking at lunar surface or planet, the object will look soft even if in focus, this is a factor of seeing & why the best approach is to shoot video & then use Autostakkert to stack the best images. Unless you are extremely lucky, single exposure images on planets & lunar rarely result in a sharp image. A Bhatinov mask can be used for planetary imaging as well, just focus on a bright star first using the diffraction spikes generated by the mask & then slew to your target of choice. However, even without a bhatinov mask, you should be able to achieve good focus, it just won't look like good focus on your screen, this is a factor of seeing.. your eye/brain is able to deal with this (organic adaptive optics... lol) & you will see your image flutter in & out of focus. Your camera sensor well, on a still shot, you just have to be lucky whereas with video, 30 seconds will capture you a thousand or so frames & give you plenty of in focus frames to stack. You can then further sharpen using wavelets in Registax.

If you are trying to focus on a deep sky object, you really need to be using either a bhatinov mask on a bright star or alternatively, capture software that features a focusing component.'

I have found that trying to focus stars in a camera by eye is very hit & miss, generally more miss...

If you are running out of travel on your focuser, well, that's a different issue..

Hope this helps

[truss]

The focuser definitely isn't running out of travel, there is plenty either side of the focus.

Visually the Moon is razor sharp through any of my eyepieces, Jupiter is the same, stars are pin point sharp. The issues are when imaging, and the Bhatinov mask doesn't help. Using my 7D, I can throw it into liveview and zoom in on screen to a star and get the focus to where it "appears" to be right, only to have any imagery come up soft.

I'm beginning to think the camera might be the issue, it could be over-sampling. And pixel binning to counter it is something I don't even know if it's possible in the Canon 7D.

[Outcast]

Does your image look a little like the one below?

This was taken through an 8" SCT with an EOS1100d, you will get star bloat on brighter stars without a reducer...

Regards binning, depending on what you are using, it can be done in post processing.

Cheers

[truss]

It does actually, and I now have a f/6.3 reducer in the kit (weather hasn't allowed me to test it out properly), so I will give it a proper go hopefully tonight.

Binning in post, I'll have to look into that (Canon 7D is the weapon of choice).

[Outcast]

What are you using for post processing?

FWIW, I use DSS & Startools

Can't see any options for binning in DSS but, I know Startools allows you to do binning at different rates.

Not sure about other post processing software... also, not 100% sure on this next bit but, I think some capture software allows you to do binning as well... I use EKOS & I have a vague recollection that I can set binning, even for my EOS70d...

However, I don't believe there is any such option in the Canon settings in the camera...

Cheers

[truss]

I'm still tossing up what tools to use, work has been that busy I haven't had a chance to sit down and have a play with the various offerings.


There are no binning options in the 7D, from what I can tell Canon have never had it in any of their DSLR's.

[Camelopardalis]

The easiest way to "bin" is just to resample in integer scales, so if your image is 2000x2000, resample to 1000x1000 for bin 2x2.

Also check there's no slop in any of the connections between the rear cell of the scope and the camera. Any tilt introduced there can result in only a part of the image being in focus with the rest being out.

[truss]

Zero slop through the rear cell and camera, it's all nice and tight.

Thanks for the tip regarding "binning", I hadn't considered resampling at lower resolutions as being a form of it.

[Camelopardalis]

At 2m focal length, is more than likely it’s just soft seeing. If you centre a bright star and go into live view with 5x or 10x, what does the star look like?

[truss]

I use liveview pretty much the whole time the camera is on the 'scope. At either zoom level, stars aren't pin sharp, rather soft balls that are close to focused but not quite there. That then translates to recorded imagery. The issue persists regardless of seeing conditions. But due to weather (and lack of dew shield), I haven't had a chance to fully stretch the OTA with the reducer/corrector fitted.

[Camelopardalis]

So the thing you've got to bear in mind is that at f/10, the diameter of the airy disc is quite large. If you're expecting stars to be small pinpoints, you'll be disappointed. Bahtinov masks are pretty effective at getting good focus. The stars will be what they will be.

With the reducer, you'll get smaller stars...because the diameter of the airy disc is proportional to f-ratio.

The reduced focal length will probably be a little more usable, all things considered, and your exposure times will be need to be shorter to achieve a similarly bright image, compared to f/10.

A disadvantage with the reducer is that the illumination suffers across the field of view, especially with a relatively-large APS-C sized sensor.

This game is all about trade-offs :shrug:

[truss]

I think the airy disk of this particular telescope is too large for the sensor in the 7D, hence the lack of razor sharp stars. It is why I was thinking the camera is oversampling.

Seeing as the camera is becoming more troublesome from a noise and hot pixel perspective (all correctable in post), I'd need something like a 5D MkII to come close to being a better match for the SCT.

[Camelopardalis]

Except on the best of nights, the 7D will be oversampling. It would work well on a nice refractor well below 1000mm focal length ;)

If you're finding it troublesome, then go for a cooled astro camera. The noise can then be reproduced in dark frames and subtracted from the images (calibration).

An 8" SCT is not designed to work with a full frame camera, and you will see pretty dramatic light drop off in the corners of the image, so be prepared to crop heavily.

[truss]

A cooled astro camera is on the shopping list, however money is being put aside for a EQ mount first, looking at a NEQ6 Pro. Having said that, which of the current crop of cooled cameras has a pixel size in the 8-10 micron range?

[Camelopardalis]

No CMOS cameras that I'm aware of have pixels that large...you're better off reducing your focal length... f/10 won't show you what you hope for.

[truss]

Hence the f/6.3 reducer/corrector. My one has turned out to be one of the last Japanese made units, very sharp bit of kit.

[truss]

I found a Canon 5D MkII today for very cheap (<$300) and I had a small window to test it this evening, boy what a difference between it and my 7D. The difference is so profound, the 7D has been permanently taken off astro duties. The pics are nothing special, just quick test shots. Once I get a clear sky, I'll be able to take some proper shots.

The camera has 24,000 shutter actuations, barely been run in so to speak.