From: "Herb Kasler" 
To: "Jerry B. Hillman" , 
Reply-To: "Herb Kasler" 



Jerry,

Just to reiterate what everyone else is telling you, you should not give up
on your mirror until you make a proper cell for it. 3/4" thick is the
thinnest 16" mirror I have heard of anyone making (although I am
relatively inexperienced). At F/4.5, you're probably only 1/2" thick
in the middle, maybe less if you spent a lot of time grinding. That's
really pushing the envelope. If your mirror is going to work at all, it
will need a good support system. What you currently have is not a good
support system. Any rigid,  static system of supports like the one you have
will in effect have not the 44 points you made, but only three points. A
rigid, flat body like your mirror will rest on only the three highest
points. The rest of them might as well not be there. So what you have now
is a three point cell with the support points randomly distributed. The
beauty of flotation cell designs is that they automatically distribute
exactly the same amount of force to every support point under the mirror.
But just to test the thing you might not need to get so complicated. You
could do someting as simple as putting a piece of  high-napped outdoor
carpeting behind your mirror. Although this does not have the elegant
distribution of force that a flotation cell has, and it won't hold its
collimation as well because of its mushiness, it does provide many
thousands of flexible support points, and should not cause any astigmatism.
You could also draw a big spiral bead of something like Silly putty on your
back support and lay your mirror on that. The putty will flow until it has
an even distribution of wight on it. Again, this would not be a permanent
solution, but it would allow you to reaonably test the mirror. As an aside,
you should also be providing good support to your mirror while you are
doing the Focault test. If you just stand that baby on its edge, it will
probably sag quite significantly. I made a test stand for mine by making a
square frame about 2.5" deep with teflon pads all over the inside of
two adjacent sides. Any mirror you put in there (with the box standing on a
corner) will be supported at two points 90 degrees apart, which will tend
to cancel out any deformations from sagging. The mirror was tilted slightly
face-up, so the back of the box was covered with soft adhesive felt
furniture pads. These are flexible enough that the support of the mirror
will be distributed to many points. I took all these precautions for a
mirror that was 1.5" thick, so you should definitely do at least as
much for one that's half that thickness.

Good luck,


Herb.

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