[ATM] Auto Collimation Flat - not cored

[Dominic-Luc Webb]

I reported years ago (about 10-12 or so) that my full
thickness pyrex 8" F/8 accidentally turned into a flying
saucer when it fell out of its cell, and rolled down the
tube. I lifted the front of the tube trying to keep it
from ending up on my hard concrete floor. It had already
rolled farther than expected, and my lifting ended up
catapulting the mirror right out of my balcony. It nonetheless
hit concrete, way down on the road below.

What to do at this point? Well, I quickly decided to savor the
moment and enjoy watching the flight of the only 8" F/8
parabolic flying saucer I am likely to ever see. After some
moments of collecting my thoughts and realizing I now had an
excuse to make another mirror (yippie), I went down and fetched
it. Amazingly, only a chip on one side was lost. I still have
and use that mirror, although, I am quite sure the figure must
have changed considerably. I have no before and after pics
of the figure for comparison, but I am sure I would not have
aluminized it with the figure it currently has. I could be
made to believe that a hole, or a crack, etc can change the
figure of a flat.

However, I also know that manufacturers of glass filters produce
large precision flats that are cut into the desired sizes. I am
told these are very large sheets and many disks are cut from a
single sheet.

Dominic

On Thu, 9 Aug 2007, Francis J. O'Reilly wrote:

> Shane,
>
> Have you considered the use of a beam splitter for the autocollimator? I would be hesitant to core a perfectly good flat. My concern would be that cutting into the surface of the flat would release stresses in the glass if there are any, and I believe that there usually are even in the most finely annealed glass, and cause a change in the surface. A beam splitter will allow part of the light to be reflected at ninety degrees for your viewing (or photographing) and analysis.
>
> Hope to see you at Stellafane this weekend.
>
> Francis J. O'Reilly
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