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C11 Collimation Proof with ASTAP

Han K. Astrometric STAcking Program (ASTAP) Acquisition Deep Sky
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Jeramie avatar
Jeramie avatar
Greetings, 

During a recent processing attempt I noticed that for the amount of data that I had collected my end result didn't look as crisp as I thought it should. I thought maybe my collimation was off and was part of the issue potentially. 

I loaded a few of my subs into ASTAP Image Inspector and they all report to have almost no tilt, but does this confirm that the scope is well-collimated?

Appreciate any comments on this. 

Himanshu Pandey avatar
Himanshu Pandey avatar

I think of collimation as fine focus and a separate issue from tilt. You can be a little off on collimation and still believe you are in focus but if you take the time to look into and then collimate the scope you get a sharper image. Perfect tilt does not mean you have good collimation.

Concise
Jeramie avatar
Jeramie avatar
Is there a tool that will measure collimation digitally?

I purchased and tried the HoTech device and it was very difficult to use. I found it to make matters worse. I’ve also seen the OCAL device that’s out there, but not sure it would be much better.
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Himanshu Pandey avatar
Himanshu Pandey avatar

I’m not aware of anything that could measure it directly. I have a C8 and started by searching for some you tube videos that demonstrate how to do it by using a bright star. There are a few videos on the subject on you tube on the subject and they are all helpful.

Jeramie avatar
Jeramie avatar
Yeah, been through all of those same videos. 

Appreciate the discussion.
Ashraf AbuSara avatar
Ashraf AbuSara avatar
Hello Jeramie. Tilt and collimation are quite different issues here.

There is a lot of literature talking about collimating SCTs. Depends on how precise you want to get. Simplest method is trying to center the central obstruction by defocusing on a bright start just enough to see the central obstruction. Need to point above 60 degrees and have decent seeing conditions. This will get you roughly in the ball park.

https://www.youtube.com/watch?v=EcKQFutDEAw&list=PLEuyZoVQVfi3FjCz_XXCpsFkK75S09pt7

You can further refine your collimation for DSO with a tri bahtinov mask. 

https://www.youtube.com/watch?v=rQlvowrfyYo

You can even use some digital tools to further refine it:

https://www.youtube.com/watch?v=ynsf-Ojza3Q

Now if you are doing planetary work, then you will really need some further refined collimation methods. My favorite tool for that is Metaguide:

https://smallstarspot.com/metaguide/

To use metaguide, you either need a very small pixel camera, like 715mc or a barlow to visualize the airy pattern. 

Hope that helps,

CS!
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Ruediger avatar
Ruediger avatar

Hi all,

there is a tool to measure collimation “SkyWave“- it is not cheap, but delivers excellent results. I use it for my PWI CDK14 with excellent reuslts. To get an overview:

CS Rüdiger

Vendor: https://www.innovationsforesight.com/aitelescopecollimation/

CN infos:

https://www.cloudynights.com/topic/757247-ai-based-wave-front-sensing-and-collimation/

Tony Gondola avatar
Tony Gondola avatar

No, all it’s telling you is you have no tilt. The easiest way to judge collimation is find a medium bright star, center it and rack slightly out of focus, I mean just enough to show the donut, you might have to digitally zoom in to see it well. Is it concentric? If it is, you are in collimation, if not, you are not. If you make adjustments, make sure that you re-center after each one. You don’t need any fancy software or a dedicated device to tell you if collimation is good or to fix it if it isn’t, not with an SCT.

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Ruediger avatar
Ruediger avatar

Tony Gondola · Aug 20, 2025 at 05:33 PM

No, all it’s telling you is you have no tilt. The easiest way to judge collimation is find a medium bright star, center it and rack slightly out of focus, I mean just enough to show the donut, you might have to digitally zoom in to see it well. Is it concentric? If it is, you are in collimation, if not, you are not. If you make adjustments, make sure that you re-center after each one. You don’t need any fancy software or a dedicated device to tell you if collimation is good or to fix it if it isn’t, not with an SCT.

Hi Tony,

For a rough collimation you are right. But a perfect donut does NOT mean you have the BEST collimation and best imaging quality. E.g. my CDK has an asymmetrical donut when perfectly collimated. This is also confirmed in the images and measured by the software. A symmetrical donut is a start point but potentially not the best result.

CS

Rüdiger

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Tony Gondola avatar
Tony Gondola avatar

But the OP has an SCT for which the advice applies.

Please explain your comment about your CDK. That’s the first time I’ve ever heard anyone suggest that a non-symmetrical pattern from the center of the field was a good thing. Not saying that it’s not possible because I have no experience with corrected Dall-Kirkham telescopes but I’d like to understand why that would be the case.

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Ruediger avatar
Ruediger avatar

Hello Tony,

it also applies to SCTs.

That having perfect symmetric donuts will result in perfect star shapes is a commonly spread false assumption. For that Dr. Gaston Baudat has published quite a lot to read through. If you are interested in the background, you will find all the information when following the links given above.

But usually the symmetric donuts are fair enough for a decent imaging result. Especially, when you try to optimize your collimation without software and only based snapshots in focus. That is why working with extra-/intrafocal donuts was common until now. With software based analysis you can optimize beyond “donuts symmetry” and optimize for result.

CS
Rüdiger

Update: you may read this post:

https://www.cloudynights.com/topic/757247-ai-based-wave-front-sensing-and-collimation/page-8#entry11774624

It deals with additional effects which show an asymmetric donut though it is perfectly collimated and only diffraction limited (best result).

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Jeramie avatar
Jeramie avatar
I tend to agree with @Ruediger on this and this is why I'm looking into a tool that provides feedback/confirmation. 

Below is an example of the stars from a recent project on M57. I have 30 hours of data collected on this project and the data is, in my opinion, garbage. I collimated frequently using the donut method, and this is the result. 



Very bad star shapes overall, and even with NxT/BxT they can't be fixed. My thought is that if my stars are this bad, other objects will be as well to the point that the overall project is impacted. Having a system that confirms collimation night-to-night will help to prevent this moving forward. 

It's also important to note that this is very long focal length and collimation becomes critical. If I were using my widefield FMA230 or Hyperstar on the C11, I might be able to get away with less-perfect collimation.
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Tony Gondola avatar
Tony Gondola avatar

Ruediger · Aug 22, 2025, 01:37 PM

Hello Tony,

it also applies to SCTs.

That having perfect symmetric donuts will result in perfect star shapes is a commonly spread false assumption. For that Dr. Gaston Baudat has published quite a lot to read through. If you are interested in the background, you will find all the information when following the links given above.

But usually the symmetric donuts are fair enough for a decent imaging result. Especially, when you try to optimize your collimation without software and only based snapshots in focus. That is why working with extra-/intrafocal donuts was common until now. With software based analysis you can optimize beyond “donuts symmetry” and optimize for result.

CS
Rüdiger

Update: you may read this post:

https://www.cloudynights.com/topic/757247-ai-based-wave-front-sensing-and-collimation/page-8#entry11774624

It deals with additional effects which show an asymmetric donut though it is perfectly collimated and only diffraction limited (best result).

Ok, that’s interesting. The collimation software from Innovations Foresight can indeed be a software solution for the OP as it’s reconstructing the actual wave front to achieve the best collimation. Interesting solution, especially for all spherical optics. I’m always a bit skeptical though of motivations based around a commercial “must have” product. Maybe it’s in the Cloudy Nights thread but have users generally found that it makes a qualitative difference in their images?

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Ruediger avatar
Ruediger avatar

This company has good reputation. As far as I know, also John Hays uses their solutions e.g. on-axis guiding

My star shapes improved dramatically. I had coma when adjusted to perfect donuts. I could not get it fixed without the software, which took me less an hour.

Also in the CN threat you find dozens of positive feedbacks. It is definitely not snake oil.

Also why should people spent such a huge pile of money if it is not working?

CS Rüdiger

Rick Krejci avatar
Rick Krejci avatar

Ruediger · Aug 22, 2025, 04:41 PM

This company has good reputation. As far as I know, also John Hays uses their solutions e.g. on-axis guiding

My star shapes improved dramatically. I had coma when adjusted to perfect donuts. I could not get it fixed without the software, which took me less an hour.

Also in the CN threat you find dozens of positive feedbacks. It is definitely not snake oil.

Also why should people spent such a huge pile of money if it is not working?

CS Rüdiger

I’m just starting with Skywave and it does seem like a better quantitative way to adjust true collimation. Can’t pass perfect judgement yet since I’ve just started with it a few days ago.

When people say it costs too much, I’ll just ask them how much they’ve spent on lasers, Ocal, Cheshire sights and other devices, all of which I’ve found to get you pretty close, but ultimately not completely effective for me for camera work. Having your actual image train in there and having SW tell you exactly which screw to turn and then measuring the result to me is the best way to get results. Is it overkill? Perhaps for those of us who do long exposures with marginal seeing it is. But I like knowing I’m getting the best images I could be out of the scope I perhaps spent thousands on.

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Tony Gondola avatar
Tony Gondola avatar

I’m assuming that Skywave, while it will work, is less useful for Newtonians and that the “doughnut” approach is still valid? There was some mention of even small secondary offsets negating using the usual methods but I’m not sure it was a reference to Newtonians.

Ashraf AbuSara avatar
Ashraf AbuSara avatar
If you want very accurate collimation, Metaguide is a very good tool and is free. Some of the best planetary images use it successfully. It is a bit clunky but works. 
Jeramie:
I tend to agree with @Ruediger on this and this is why I'm looking into a tool that provides feedback/confirmation. 

Below is an example of the stars from a recent project on M57. I have 30 hours of data collected on this project and the data is, in my opinion, garbage. I collimated frequently using the donut method, and this is the result. 



Very bad star shapes overall, and even with NxT/BxT they can't be fixed. My thought is that if my stars are this bad, other objects will be as well to the point that the overall project is impacted. Having a system that confirms collimation night-to-night will help to prevent this moving forward. 

It's also important to note that this is very long focal length and collimation becomes critical. If I were using my widefield FMA230 or Hyperstar on the C11, I might be able to get away with less-perfect collimation.

Your stars can be bad from numerous other causes. This does not suggest that it is collimation. Are you using the AM5 for this image? AM5 can have a tough time at that focal length especially if the RA error is significantly worse than the DEC. Also camera fan vibrations can be a problem.  Maybe post an image of a short exposure of a rich starfield. SCTs are brutal and there are many things that can go wrong with long exposures. Tube currents are another issue.

You can also try pointing to a bright star, turn guiding on, and then open firecapture and let it run a video of the bright star. It can sometimes show you high frequency vibrations causes by the mount that cannot be captured by guiding software with longer exposures. 

Frankly for DSOs "perfect collimation" is not necessary. Most people won't have the seeing conditions to realize perfect collimation, and they are far more likely to have other problems. But when it comes to high resolution planetary imaging, getting as close to perfect becomes a must.
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