Re: ATM Mirror questions now Mirror Termal Effects

[PAugello@aol.com]

Robert:
     I don't follow this, I'm afraid.  When I first started thinking about
ATM, I wondered about this as well and convinced myself that _when at thermal
equalibrium_ a parabola is still a parabola.
     In a nutshell, the analysis I did led me to believe that since the
difference is distributed in a parabolic manner the surface would still be a
parabola -- a different parabola, but a parabola still.
     Could you outline your analysis for me?

Take care,
Peter Augello

In a message dated 97-05-12 16:30:25 EDT, RDuvall@pixworks.com (Robert
Duvall) writes:

<< The question originally was why not use plate glass for telescope mirrors.
 I said I did some calulations years ago which convinced me it was a bad
 idea for mirrors over about 8" diameter.
 
 I have revisited my formulas (which are quite simple really), and stay with
 my recommendation to avoid plate glass, especially for short focal length
 fast systems, where the problem is much worse. For f/8 to f/10 systems
 (mirrors with small sagitta), the thermal expansion/contraction effects are
 much smaller and usually acceptable. >>