Here's one for the glass pundits: I recently bought a
30 year old 6" OTA. Initially it showed a 3-symmetry
pattern in star tests. OK, so I'm thinking the three
retaining clips are too tight. I checked and indeed
the clips are fairly tight. Removing them eliminates
some of the pattern, but not all of it. I am guessing
that 30 years with tight clips has bent the mirror in
some semi-permanant fashion.
I should add that this particular style of mirror
cell had the three rear support points rotated 60 degrees
to the mirror clips -- so tight clips bends the entire
mirror, rather than just pinching it under the clips.
It is 1" thick, looks like a poured Pyrex blank.
Is it possible to "unbend" the mirror? Ideas:
(1) rotate mirror 60 degrees, re-tighten clips
and wait 30 years? Too slow....
(2) suspend the mirror on three points, and heat
to say 150F (something a little higher than any
temperature it is likely to have encountered in the
last 30 years) for a couple hours?
Other ideas (short of re-figuring)?
Thanks, - John B.
30 year old 6" OTA. Initially it showed a 3-symmetry
pattern in star tests. OK, so I'm thinking the three
retaining clips are too tight. I checked and indeed
the clips are fairly tight. Removing them eliminates
some of the pattern, but not all of it. I am guessing
that 30 years with tight clips has bent the mirror in
some semi-permanant fashion.
I should add that this particular style of mirror
cell had the three rear support points rotated 60 degrees
to the mirror clips -- so tight clips bends the entire
mirror, rather than just pinching it under the clips.
It is 1" thick, looks like a poured Pyrex blank.
Is it possible to "unbend" the mirror? Ideas:
(1) rotate mirror 60 degrees, re-tighten clips
and wait 30 years? Too slow....
(2) suspend the mirror on three points, and heat
to say 150F (something a little higher than any
temperature it is likely to have encountered in the
last 30 years) for a couple hours?
Other ideas (short of re-figuring)?
Thanks, - John B.
