[ATM] Re: Pre-stressed figuring?

[eyesky <eyesky@gmail.com>]

Well, I've gotten some great feedback on my question both on and off
list.  Thanks to everybody. =)

It was pointed out to me that a similar process was used on the Keck
telescope; nothing I have read in regards to this particular case
strikes me as being very applicable to the ATM community,
unfortunately (which is the direction of my interest at this time).
However this did lead me to a short discussion on the ATM list from
Dec. 97, and I'd thought I'd point it out. Here's the link:
http://astro.umsystem.edu/atm/ARCHIVES/DEC97/msg00678.html

There seems to be little in the way of first-hand reports of grinding
a mirror while under tension (at least that I've found so far).  There
is a note that an application of a single point force to deform a
spherical surface would result in, at best, a 1/2 wave error at a
particular radial distance (if I understand correctly).  So forming a
true parabaloid would need application of force in a more complex
fashion than from a single threaded bolt or what-have-you.  It's still
unclear to me whether these results are 'good enough' for amateur
applications or not.
Also mentioned: complex equations for the forces needed and resultant
deformations, but I haven't actually seen them in print yet.

As for an experimental setup, I imagined epoxying a ring to the
mirrors edge.  Attach a flat plate to this ring, with a threaded hole
in the center.  A fine-threaded bolt (with suitible protection against
damaging the glass) could be tightened to apply a force.  I don't know
how unifrom the pull would be on the edge in practice, though.
It seems concievable that one could unwittingly grind in asygmatism or
other unwanted optical deformaties into the glass if you weren't
careful, so there are some unknowns still.

I won't likely be testing the process myself soon, regardless.
_______________________________________________
ATM mailing list http://www.atmlist.net/