Can you get round stars from C9.25 XLT + FR/FF?

[Jammy]

I've never had any success with my C9.25 when trying to get DSO's.  I've seen images other people have taken and they have good round stars. I can never get these.

I've tried collimating the best that I possibly can, and adding the Celestron f/6.3 focal reducer but I still can't get round stars.  I could understand it if the stars at the edges had comma, but the stars near the centre are showing comma too.

I've changed my back focus both in and out to no real difference, I'm just wondering what I'm doing wrong?

I took the two attached images last night.  One shows my out of focus star from the camera, and the other image shows an integration of 120 x30 sec subs of M63 Sunflower Galaxy.  The camera is an ASI 1600mm Pro.  These are both full frame, not cropped.

When I've plate solved the individual frames for M63 they give a focal length of 1483mm.  1480mm would be f6.3 on this scope, which the focal reducer is supposed to give indicating my back focus spacing is about correct.

One thing I have noticed is that on the focus star the rings are concentric and clean, but the small reflections inside the smaller ring are not central.  Is this normal for an SCT?  I'd have thought the central reflections would be centred too.

I've wondered if the star comma was down to guiding errors, but would a 30 sec sub-frame show this much error?  Incidentally, my guiding last night was the best I've ever had and I don't think it was down the guiding.  When I use my refractor (although shorter focal length) the stars are perfectly round with this mount - NEQ6.

A few years ago I removed the secondary mirror to remove a large dark mark from it which was obscuring my views (it was a fleck of black paint from the inside of the tube).  When I removed the secondary, I accidentally rotated it when unscrewing.  Would this make a difference?  I know that the corrector plate shouldn't be rotated, and wondered is it was the same with the secondary?  I've searched online but can't find an answer to that.

I love this scope, but hardly use it nowadays because of the image quality.

Hope someone can offer some helpful advise.

Thanks

[GyCx]

Hi Jamie.

The collimation is out, the point of light should be dead center.  I have turned the secondary mirror when cleaning my C11 and it always needs collimating I would try that first. Collimating is like balancing a saucer on 3 fingers and trying to get it perfectly level : ).

This is worth a read too telescope - What are the aberrations of an SCT? And how can they be eliminated? - Astronomy Stack Exchange

Your 80ED will have a wider FOV and will be much more forgiving, the optic configurations are different too.  

But for a big SCT on a NEQ6 those stars aint so bad  I think your scope needs collimating but thats probably all it needs.   Also check your backfocus using a Focal reducer on my C11 its 105mm FR/C to Sensor 

For me it has to be a hell of a night to get 'nice round stars' using a C11 XLT on EQ6 Belt modded mount but use it for what is good at, getting nice resolution on small galaxies and great planetary/lunar scopes.  The Edge versions are really the imaging scopes we're pushing this kit really to do something it aint that great at.  I stumbled into using mine for deepsky as I started with planetary imaging,  I use pixinsight to process out most of the defects in my images im getting really good at it : )

[Jammy]

Thank you for the comments.  I've not tried imaging DSO's  with the C9.25 since I posted this.  I do now realise the collimation was off.  I've used it for planetary over the summer, with a Barlow and high power eyepiece I've managed to dial in the collimation pretty well.  I've taken some fantastic images of Mars and the Moon.

The weathers been clouds clouds clouds, but I'm tempted to give it another go at something like M1 which is probably too small for the 80ED.   We just need a decent spell of cloud free weather to try it out!