Ritchey–Chretien adjustment, image analysis

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Hi all,

Finally got some sky time for my scope - running a GSO RC-6", on a iOptron CEM26 mount, Canon 650D.

No adjustment to collimation since 2nd-hand purchase, have installed a focus/collimation plate since, also unadjusted.

Had a pretty nice run, but after stacking, I'm seeing some smearing in the 'bottom, bottom-left' stars in these images.
I was concerned it may be flexture from the backfocus etc, but different sky locations seem consistent.

De-focused stars show pretty good collimation just from a quick glance - my guess is the focuser/extension tubes being a bit off-axis.

What do we reckon?
Could well be that the camera/sensor isn't sitting in-plane.
I thought it may be that the extension tubes are angled off enough to be off the edge of the image-circle.

Bit frustrating in using a DSLR, ~8sec readoff time - maybe can run some 30sec exp's in a star-dense region to do checks between adjustments, hopefully can catch the clear sky tonight (not been good skies in Sydney lately!)

Single frames show vignetting in corners, my guess is just in having a 1.25" nosepiece on the T-ring - considering a 2" nosepiece as the focuser only has a single thumbscrew on the 2-1.25" adapter - two thumbscrews on the 2", so would get a more affirmative fixture in that case.

I've lost the links I had to some of the software that would analyse the flatness etc - if there's some suggestions?

Overall, just wanted to see what people's thoughts might be regarding the smeared stars, and the likely cause/adjustments needed.

Thanks all!

Clear skies!

[Leo.G (Leo)]

No adjustment to collimation since 2nd-hand purchase, have installed a focus/collimation plate since, also unadjusted.

Sorry, I have an RC8 and HAVE NOT attempted to collimate it because I think I'm too stupid. Mine came with a collimation plate (adjuster before the focuser) which I haven't touched since I bought the telescope used but I must check it one day because I know it has rotated slightly since I took possession of it.
If the focuser was removed to fit the collimation plate (which could be very different to what I have) wouldn't that require collimation because the plate may not be correctly set from purchase.
I am no expert, I rarely get to take any of my telescopes out and haven't been brave enough to adjust my collimation on the RC8.

[Moon (James)]

First - see if you can use a 2" nosepiece instead of the 1.25". That should help with the vignetting. Flat frames can help with that too.
RC scopes don't have a 100% flat field, so stars at the edges will look a bit elongated unless you get a field flattener. But even with a field flattener, it's challenging to get everthing all lined up perfectly so the optical axis / collimation is dead center & symerical - as you noiced with your images, even though you are not too far off.

One thing you can do is point to a bright star and go in live view mode and put the star in the center, then each corner and zoom in on it and compare the shapes. Do the same focused and defocused - inside and outside focus. You can play around with the collimation while you do that to get an idea how easy/hard it going to be to get it to a point where you are happy with the image.

Or you can do what Leo said and leave as it stands!
James