C-5 problem please help

I hope someone can help me. I have a white tube C-5 that used to show
perfect star test patterns on both sides of focus. I have since installed
Bob's Knobs, for collimation adjustments. The collimation seems to be very
good, however.
when I defocus a star I can see what looks like spider vein diffraction
patterns, running N/S/E/W. When I rotate the eyepiece or diagonal they do
not move.
The cross pattern does not show up in focused images [however the seeing has
been so bad that I can not use high enough power to see the airy disk.
Has anyone else seen this in there scope?
Thanks in advance
Steve

"stevew" wrote in message
...
I hope someone can help me. I have a white tube C-5 that used to show
perfect star test patterns on both sides of focus. I have since installed
Bob's Knobs, for collimation adjustments. The collimation seems to be very
good, however.
when I defocus a star I can see what looks like spider vein diffraction
patterns, running N/S/E/W. When I rotate the eyepiece or diagonal they do
not move.
The cross pattern does not show up in focused images [however the seeing
has
been so bad that I can not use high enough power to see the airy disk.
Has anyone else seen this in there scope?
Thanks in advance
Steve

I've seen similar things due to uneven temperature and bad thermal
equalization. I haven't seen them at 90-degree angles, though. Is there
anything streaky on the corrector plate?

Also, see if the secondary holder is off center. This is an odd problem I
have observed with my old C5 and with a NexStar 5. If it's off center, just
push it sideways until it's centered (you may have to push moderately hard)
and then recollimate.

Hi Steve,

It's possible that the secondary mirror can be slightly deformed by
overtightened collimation screws. However, it is usually visible as a
misshapen airy disc, rather than diffraction spikes. I overtightened my 2045's
screws, and it caused the disc to appear slightly triangular. The problem
disappeared when I adjusted the screws until they were just snug.


Best wishes,
Jason Glass

"Brevity is the soul of wit..."
-Polonius in "Hamlet," Act 2, Scene 2, by William Shakespeare

It's possible that the secondary mirror can be slightly deformed by
overtightened collimation screws. However, it is usually visible as a
misshapen airy disc, rather than diffraction spikes.

Hi Jason:

Exactly. In my experience, over-tightened secondary screws result in a classic
pinched optics triangular diffraction patterns, just as you observed. My guess
as to the problem with this C5? Seeing or cooldown.

Peace,
Rod Mollise
Author of _Choosing and Using a Schmidt Cassegrain Telescope_
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