A question for all SCT experts

Below is an image of a startest I did on an SCT that appears to have an optical defect. This tube apparently used to be excellent according to the owner but something happened to it. To me it looks like it has a pinched primary or secondary, a bad zone or turned edge, but it might be as simple as rotated corrector plate (wrong indexing). It's not mine so I can't take it apart but I can advise the owner. I need you experts to help me pinpoint the issue (and the star images... ). It is not thermal issue, the tube was outside at 60ºF for two hours prior to the test, I also flipped the tube upside down and the pattern flipped with it. I was using an artificial star only 65 feet away so please ignore the Spherical error, well, that and the dust on the eyepiece in the picture. No diagonal, just eyepiece and p&s camera. Real star shows the same defect. Collimation is very close but it is difficult to get it dead on because near focus not only the rings become square but the airy disk looks like it is broken to pieces. At perfect focus, the airy disk looks nice but lots of haze around it distributed unevenly. In and out focus might be flipped in this test, I'm not sure.

Thank you in advance for your help!

oren.

Attached Image:

Hi Oren,

I agree this looks like either thermal or pinch, and you seem to have eliminated thermal already. It involves a very small part of the circumference of the optic, and I don't really see how to do that to the primary or secondary (usually these SCTs don't have edge clips and such). I'm thinking it might be at the corrector plate, where perhaps there are 6 screws and one of them could be wildly over-tightened. The corrector plate does not have much power in these scopes, so it would be difficult to make much distortion in the wavefront, unless it was really, really tightly pinched. Also there are often some sort of cork pads, etc., supporting the corrector -- you might see if one or more has been displaced, gone missing, or otherwise fouled-up.

Usually the edge of the primary is free and unsupported so difficult to see how a pinch would occur there. Secondary mirror is usually bonded to a metal disk -- again difficult to see how a pinch could suddenly appear there.

I might not completely dismiss thermal. Even if it is outside for a couple hours, you can still have thermal issues if the temperature is dropping. Testing the scope on its side will exacerbate assymetries from thermal stuff -- making patterns similar to these. It is also possible that the patterns temporarily imprint themselves on the optic, so flipping the scope over is not 100% test against thermal. For example, if warm air pools at the top of the tube (scope on side), that area of the primary will also be warmer than the rest -- an effect that flipping the tube will not instantly fix. I might rather see a test on a real star at the zenith (though I realize that is not always convenient or possible) -- that might better rule-out some strongly assymetric pattern from thermal issues.

Good luck! - John B.