Re: ATM Vacuum Pans. _ Corrector size?

[Aplanatic@aol.com]

Stuart Field wrote:

> If this gap really exists, and the glass is really not parallel to the face 
> of the pan, then it is contacting mainly at the inner edge of the pan's 
lip.  
> Thus the shape and accuracy of the inner edge of the lip would become
> important.  So even if the face were perfectly flat, a somewhat oval inner
> lip would lead to astigmatism.

Since this discussion has turned to the control of astigmatism during the 
fabrication of corrector plates, I'll throw in a couple more ideas.

Regarding the issue that Stuart raises above, I carefully smoothed the 
existing bevel on both the inner and outer edges of the lip using 600 grit 
sandpaper, working at it until both edges were very smooth to the touch.  
Each time I placed the corrector back onto the pan, I inspected the layer of 
grease formed between the pan and the glass with a 15x loop, looking for 
foreign matter trapped there.  A human hair is roughly 75 microns in 
diameter, or about 150 waves, and if trapped between the corrector and pan, 
would cause considerable grief.

Some of you must be wondering how on earth this method can ever work.  Well, 
in hindsight it does seem like tempting the Gods.  But fools rush in where 
Angels fear to tread, and very often come out the other side with a 
serviceable corrector plate. <g>

Some things that should help:

During polishing, rotate the mirror on the pan quite often, and in a pattern 
that minimizes astigmatism if the pan is not flat.  This requires that the 
vacuum be released and reformed, but this is not a big deal at all because 
the plate is always deflected, during pulldown, by the same amount.  In my 
case, the corrector was quite thin, requiring only 1 PSI to deflect the 
plate.  It so turned out that this pressure, which holds the glass to the 
pan, was insufficient to prevent it from moving during polishing, thus 
requiring a bead of RTV along the outer edge of the pan, in a cut rabbit, to 
hold the plate stationary.  This proved to be quite annoying and time 
consuming.  So, I now recommend that a thicker plate be used, requiring about 
3 to 4 PSI to deflect.  I believe (from some tests) that this should be 
sufficient to hold the glass against the polishing forces, at least for a 10" 
corrector.  If not, restraining clips that do not pinch the blank should be 
tried in lieu of the RTV.

Make sure that the pan itself does not deflect during polishing, or if it 
does, make sure that the undesired deflection is randomized by frequent 
rotation.  This is essentially the technique that is used to make large, thin 
mirrors.  The mirror is placed on a piece of carpet or other compliant 
material during grinding and polishing, and the mirror is frequently rotated 
in a quasi-random pattern to avoid astigmatism.  If you make a relatively 
thin pan as in my case (0.85" thick aluminum, 0.15" high lip, 10" diameter) 
you need to treat it just like a thin mirror blank.

Many, many fine corrector plates have been made by amateurs by this method.  
Persistence and intelligence can overcome many obstacles that are placed in 
the road in front of us, but in order to do so, one must have an appreciation 
of the difficulties along the way.

Dave Rowe
Torrance, CA