Coma Corrector. Further info

[solissydney (Ken)]

After having filed off 1mm from the back of the Canon T-piece thereby making the distance to the C-mos smaller I took another image for comparison of coma with my 12 inch F/5 using my Canon 5D. I still have elongated stars on the upper left. I wonder why.
Comments please on the difference regarding coma and vignetting.
Please see "coma corrector" from my earlier post down about 12 posts.
I don't know how to copy the other 2 images to here.

I found the image taken prior to filing off 1mm from the T-piece, placed on the right side.
I made the image lighter and smaller. Still think the come has gone?

[multiweb (Marc)]

Your coma is gone. This is field rotation. Your guide star is likely to be in the bottom right part of your field which is the sharpest.

[solissydney (Ken)]

You are right about the guide star Marc
I was only able to find one star on which to guide on.
So, in future, I must do my best to use a guide star above or below, rather than one on either side, of the chosen object in order to avoid field rotation.
Thanks

[multiweb (Marc)]

Originally Posted by solissydney

So, in future, I must do my best to use a guide star above or below, rather than one on either side, of the chosen object in order to avoid field rotation.

Well.. it's not always possible but yeah as close to the center of your field as you can. If you can't then try to reduce your subs time or refine your polar alignment.

[troypiggo (Troy)]

Best to get good polar alignment to minimize that rotation.

[solissydney (Ken)]

I doubt it can be polar alignment problem from my permanent post in my shelter in the garden. But, I shall check it of course.
Can we conclude that by moving the coma corrector 1mm closer to the cmos, using my Canon 5D, has solved the problem of coma?
Thanks for the replies.
Ken

[multiweb (Marc)]

Originally Posted by solissydney

I doubt it can be polar alignment problem from my permanent post in my shelter in the garden. But, I shall check it of course.
Can we conclude that by moving the coma corrector 1mm closer to the cmos, using my Canon 5D, has solved the problem of coma?
Thanks for the replies.
Ken

To be sure set the scope on a uniform starfield devoid of nebulosity and do a 20min exposure. That will rule out (or not) field rotation.Then you can take it from there.

[solissydney (Ken)]

How about this 20 min image taken with the same 5D through an 24 mm lens piggy backed?

[White Rabbit]

I'd have to agree, you seem to have fixed the coma issue. Which part did you file down and what did you use to file it down? I'll need to do this for mine.



Thanks

[multiweb (Marc)]

Originally Posted by solissydney

How about this 20 min image taken with the same 5D through an 24 mm lens piggy backed?

You won't see field rotation as easily with a large image scale in a widefield. You need to go to the FL you had on your M8 shot to see it.

[solissydney (Ken)]

Doing that to-night Marc. Good clear sky to night
I filed 1mm off the flat side of the T-piece using just a file abour 25 cm long.
Not too laborious.

[solissydney (Ken)]

Problem solved.
The cause of my elongated stars were caused by end-slob if the Dec worm on my mount.
See Equipment discussions: EQ6 Pro Voes
Thank you
Ken