Re: ATM plate glass

[Dominic-Luc Webb molmed <Dominic.Luc-Webb@molmed.ki.se>]

On Fri, 20 Apr 2001, Kenneth Scharf wrote:

> So whats wrong with it (i.e., plate glass)?
>
> 1: you must let the mirror cool down between a
> polishing session and a star test (longer than with
> pyrex)

As I noted with Anthony Stillman's obsidian (a very poor,
though pretty, material for a mirror, it seems) primary at
Chabot, actually touching the mirror causes a momentary
deformation that is clearly visible on a Ronchi tester,
but one can see the wave form returning to stability
within a few seconds. I was not able to walk back to the
tester after touching the side of the mirror fast enough
to see anything more than the final settling of the
figure. This was something like a 4-6" diameter piece no
more than about 3/4" thick. I am now seeing something
similar for a 7.25" diameter 3/4" plate glass I am now fine
grinding as an F/1.0 (i.e., very thin in the middle). I
suspect the time for these thin plate glass mirrors to
equilibrate is quite rapid. Perhaps the precision of the
desired figure affects this, but I don't imagine any
problem getting to something like 1/8 wave.


> 2: the mirror may take longer to adjust to temperture
> changes after sundown.

I am not so sure. I think there is a trade off here. The
plate glass I have seen is much thinner, and as a consequence,
should equilibrate faster than thicker glass. Pyrex has
very little change in figure with a change in temperature,
but I would guess that the standard thickness of pyrex
mirrors causes them to take longer to equilibrate and
adjust back to their "native" figure, however small the
change may be.

As you point out, Kenneth, and I agree, these both are
acceptably managable issues.

Dominic