Coma detection as a collimation aid

Got an idea for something that SharpCap should do? Share it here.
Forum rules
'+1' posts are welcome in this area of the forums to indicate your support for a particular feature suggestion. Suggestions that get the most +1's will be seriously considered for inclusion in future versions of SharpCap.
Post Reply
Borodog
Posts: 330
Joined: Fri Jan 01, 2021 7:25 pm

Coma detection as a collimation aid

#1

Post by Borodog »

There is already a collimation tool that uses star sizes for non-coma-corrected Newtonian telescopes. However, it occurred to me that SharpCap is already very close to being able to detect and quantify coma as a collimation aid. SharpCap already detects stars across the field and calculates their COG and integrated total brightness. It should be relatively straightforward to also calculate their 2nd moments relative to their centroids about the X & Y axes, and from those their principal axes and moments. The principle axis with the highest moment is the coma axis, if any, and a line could be plotted through the star's centroid, parallel to this axis, and perhaps with a length proportional to:

(maximum principal moment / minimum principal moment) - 1

When there is very little difference in the 2 principle moments, the line is length zero, but when coma becomes significant you will get a line proportional to its severity. All you need is a suitable scaling factor out front, perhaps 5 or 10 pixels.
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Coma detection as a collimation aid

#2

Post by admin »

Hi,

I personally found that the current coma based collimation aid wasn't as useful as I'd hoped, which partly explains why it has not been developed further. You might well be right that adding detection of the angle of extension of the object and plotting that may bring enough extra information to be useful for the final tweaks to collimation.

Coincidentally, I also have a nice F4 newtonian sitting under my desk at the moment - I have been playing around with camera based collimation techniques using the view of a camera with a lens placed in the drawtube :) If we get a clear night I can find out if the attempts were any good!

cheers,

Robin
Borodog
Posts: 330
Joined: Fri Jan 01, 2021 7:25 pm

Re: Coma detection as a collimation aid

#3

Post by Borodog »

That is a very interesting idea. I have been very frustrated with lunar imaging with my 10" ~f/4.7 Dob + coma corrector. I find the 3-way collimation & alignment (secondary, primary, focuser) of the Newtonian design to be extremely fiddly. Having a lensed camera in the draw tube would allow you to adjust everything while watching the effect on screen.

What kind of camera and lens are you using?
Borodog
Posts: 330
Joined: Fri Jan 01, 2021 7:25 pm

Re: Coma detection as a collimation aid

#4

Post by Borodog »

A related idea is something like ASTAP does to automatically determine curvature and tilt; SharpCap already has all of the basic parts needed to do this automatically.

If you prepare a series of images of a rich star field varying in focus by fixed steps, ASTAP can determine the best focus for each star and from that independently determine the field curvature and tilt. SharpCap could automatically do the focus sweep using your last focus scan settings. Combine that with the coma measurement and you have a complete tool for assessing the curvature, collimation, and sensor tilt of the imaging train. This would allow extremely precise tuning/optimization of the image train entirely withing SharpCap.
User avatar
admin
Site Admin
Posts: 13177
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Coma detection as a collimation aid

#5

Post by admin »

Hi,

another nice idea - thanks for the suggestion :) I will take a note of this in my 'big book of SharpCap ideas' :)

cheers,

Robin
Post Reply