A second pair of Ontario eyes might not help here: to me the stars are either very fine in the center or fine enough that if you can achieve the same roundness across the field, edge to edge, you will probably be very satisfied :-) Regardless, your path to happiness (in the short term ;-) is to get round stars near to the edges.
I suspect that there is a utility out there that can scrutinize stars for roundness down to the pixel image. If not, there should be, and if a utility can diagnose, that would be great.
I think that more difficult feats have been accomplished.
Are any CNers aware of such a tool??
Not sure how this helps at all fixing the issue here bud?
Unfortunately the image posted is a low resolution and on my screen it shows round center stars but with a slight donut effect (slightly darker in the centers) which could be a focusing issue that would amplify the corner issues further than normal.
All corners in your picture show slight elongation with 2 of them showing definite coma. This would to me show a few areas to check and collimation is an easy one to check with a reflector.
Any coma issues at all (with a properly adjusted field flattener/coma corrector) should be re-verified by simply verifying collimation is good to start with. Once you know its good then you can rule it out. It is not uncommon for all reflectors to lose collimation at any time... it just happens when moving them etc.
I just had a very similar experience with a refractor but unfortunately it was unfixable (bad collimation and no adjustability) and it was replaced by the manufacturer. I adjusted the back spacing on the FF back and forth and even sent them a Bahtenov mask focus photo to verify to them the scope was indeed in perfect focus when taking photos accompanied by the example star shots.
So, really what I am saying is... first check collimation to rule it out, then when imaging, look at your target and make sure it is in perfect focus and not just round stars in the center... because, stars can be out of focus slightly and still be round which will amplify the corner star issues further... and then adjust back focus to get rid of any elongation and what is left is maybe some residual coma or a perfectly flat image. You may have a bad coma corrector or multiple issues that reduce the ability of the corrector to do its job perfectly.
There is no tool available for checking everything, but you can get expensive software that can tell you if your sensor is tilted and you can use software or masks to see if you are in perfect focus.... coma can be checked by a $30 tool and elongated stars are variables that are trial and error based on back spacing.... but alas...no magic tool for everything. It can be a frustrating hobby when adjusted properly isn't perfect!
Cheers, I hope this free checking tool helps!
Edited by Kevin_A, 30 October 2020 - 11:08 AM.