Vintage 90mm f=1000mm Orion Skyview achromat OTA

I am the second owner for this scope. I bought it to keep as a collector's item. But when I decided to sell it and tested it, it performed reasonably well, but its star test had an issue. I wrote about it in the cloudy nights forum, but so for I got few helpful comments.
Please read the cloudy nights short thread, below, and give me your feedback.
Regards.
http://www.cloudynights.com/ubbthreads/showflat.php?Cat=0&Number=6584666&page=0&view=collapsed&sb=5&o=all&fpart=&vc=1&PHPSESSID=

That is interesting! I've had both the SkyView 90 (the one made in Taiwan from 1997-2000) and a few of the older Meade 395 90mms...The best 90mm I've peered through as a doublet achromat indeed was the SkyView 90. Mine never had this issue. Both of the Meades were sharp up until 110x-125x and I would blame the not-so-well-crafted hard plastic lens cell. Hope you can fix the problem soon; those scopes DON'T come around very often.

Hello, Saied.

A triangular fresnel pattern may indeed indicate pinch astigmatism in the system. If you've confirmed there are no light path intrusions (screws, et.al) causing diffraction, then pinch astigmatism is likely. In that case, I would expect one of the objective elements to be pinched.

Have you confirmed that the objective is the source? If you can, try rotating the cell assembly while viewing the fresnel pattern (defocussed star test). An extra pair of hands (another person) will make this easier. If the triangular pattern rotates with the objective, then you know the objective is the source of astigmatism.

It's possible that the diagonal is pinched. Astigmatic diagonals usually display an elliptical pattern of astigmatism, but a three-point pinch is still possible. Try another diagonal, or try without the diagonal.

I also recommend backing off the retaining ring to allow for the slightest rattle, then leave the ring loosened (do not snug it back against the objective). Once done, check your fresnel pattern again. If you're still seeing pinch astigmatism, perhaps the front element remains pinched - even if the rear element is not - perhaps due to a lodged spacer (not likely) or a second retaining ring (even less likely), or due to the element's fit within the objective cell (maybe lodged/pinched). If you must, back off the retaining ring a good bit more to check for "breathing room" for both elements. Hold the cell assembly "face down," lower it to a "pedestal" topped with sterile cotton or white unscented facial tissue (something scratch-free) and make sure the crown-spacer-flint stack does indeed elevate within the cell. If you confirm neither element is pinched, re-set the retaining ring (again, allow for the slightest movement/rattle).

If none of the above works, let us know.

Best wishes and kind regards.
-Dan

It is my guess the triangular patterns were created in the fabricating process or the glass itself is defective.