3 lobed stars - Pinched Optics?

[JonathanA]

Hi,

I've been struggling for a while with my comethunter scope (ES MN152). It is a Maksutove Newtonian  I am fairly confident my collimation is good, the corrector and primar are aligned, and my focuser is solid but I keep getting 3 lobed stars in images. It is worse in some areas of the field than others. Suspecting mirror pinch, I have backed off the mirror clips so that I know that they are not touching the surface of the mirror. Previously the mirror had a lot of lateral movement in the cell so I have also shimmed (between the side of the mirror and the wall of the retaining clip) with soft fabric so that it moves less. Could this be the cause? Also, I think I have possibly overtightened the secondary retaining ring that holds it to the corrector plate but would this give 3 lobed stars? I would expect some other pattern if the pinching was circular...?

I'm really running out of ideas here...and need an expert.

My specific questions:

Could this aberration even possibly be caused by the overtightened secondary retaining ring on the corrector?

Could overtightening the setting screws on the secondary cause this?

Is it most likely to be the shims on the primary - It would surprise me if they could exert very much pressure?

Maybe this is just coma and I'm barking up the wrong tree........

Hoping for help.......

 

[Peter Drew]

Three lobed star images are the classic sign of a pinched optic, most likely the primary although you seem confident that yours is not tight. A pinched secondary would usually introduce astigmatism. The corrector is quite thick and has a weak but important optical effect and can stand a fair amount of pressure. Your image shows more like three stars rather than just one three lobed appearance, a similar effect can be caused by thermal problems but this would be easy to detect visually as they would be in motion whilst you look. I can only suggest you back off any retaining or adjustment features that could be exerting undue pressure.

[JonathanA]

Thanks, Peter.

I backed off the clips on the primary even further and accepted that I will get mirror shift depending on pointing direction and that I would need to do a recollimation prior to an imaging session once I was lined up on the target. I know this is good practice anyway.

Some astigmatism remains though. (I think at least some of the 3 balls effect seen in my earlier post may have been partly down to insufficient cooling - the remaining distortions are much sharper and more blade like than ball like) The remaining distortions of stars were mostly restricted to the edge of the field and the effect of the distortion seems to have radial symmetry around the field. From this I have reached the tentative conclusion that it may well be overtightening of the secondary retaining ring that is causing the problem. Also, I noticed that a couple of the screws holding the peripheral retaining ring of the corrector were more tight than the others so I have backed these off too.

I feel that the situation is improving and I hope that these last tweaks will put me right. I'll report back after further testing!