Re: ATM Vacuum Pans. _ Corrector size?

[Aplanatic@aol.com]

Kevin McCarthy wrote:

>       This won't work.  To avoid introducing astigmatism of less than one
>  wave, you want an annular supporting surface whose "planarity" is less
>  than one or two waves.  Conventional machining just doesn't cut it here
>  (0.001" is 50 waves).  Neither will "compliant" (ie, silicone, O-ring,
>  or foam rubber) solutions work.  In this case (despite encouragement
>  from ATMIII), the local variations rubber compliance lead to plate
>  deformations.  The proper support is a very flat annulus, preferably of
>  metal.
<snip other great advice>

Let me add just a note or two to Kevin's excellent post.

My experience with the Slevogt corrector provides corroborating evidence 
about the flatness required of the vacuum pan.  The first time through I was 
pretty sloppy about lapping it flat.  To lap it I used an old piece of 
aluminum plate which wasn't at all flat to begin with.  Thinking that 
symmetry and randomness would come to my rescue, I lapped the pan on this 
plate for a while, rotating the pan a lot during the process.

After "finishing" the corrector plate and throwing it on the test stand with 
the mirrors, I was horrified to find that I had ground and polished in 
massive amounts of astigmatism.  At focus, the astigmatism caused flares in 
the image that extended out perhaps 60 microns from the center.  Bad.

So, I made a two ball spherometer and measured the flatness of the pan lip. 
It showed about 400 microinches of waviness as I rotated the spherometer 
around the lip.  I bought a sheet of 1/2" thick glass, put it on some 
carpeting and spent hours getting the sheet of glass flat.  I then re-lapped 
the pan on this glass until I could no longer detect any variation in the 
height of the pan lip.

To make a long story short, I spent many hours re-polishing the corrector on 
this re-lapped pan, eliminating the residual spherical aberration of the 
optical system at the same time I removed the astigmatism.  In the end, it 
would have been better to start over and regrind the corrector, but I enjoyed 
watching the corrector slowly conform to my will.

During this second attempt at figuring, I made darn sure that the glass was 
in full and intimate contact with the glass and that the vacuum grease was 
applied in a very thin and uniform manner.

"Paranoia is the friend of the optician."  At least when making a Schmidt 
plate.

Dave Rowe
Torrance, CA