[ATM] Thermal mirror deformation

["Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>]

Hello,

Inspired by the recent thread on under/over correction of a mirror due to
radiative cooling, I have done some simulations with a PDE solver and
written an article about it. Please have a look at
http://home.hetnet.nl/~artm/atm/articles/cooling.html
Iwould very much appreciate any comments: there might be wrong assumptions
or errors.

In summary, heat radiation from the front surface of a mirror may, depending
on the situation, indeed cause a temperature difference of a few tenths of a
Kelvin. As a consequence, especially largish plate glass mirrors will suffer
unacceptable deformation. In one simulated case the additional sag of
spherical bending of the mirror is as large as 700nm. Of course this will
only partly show in the eyepiece, since most of it can be compensated by
refocusing. But anyhow, my findings are way larger than I would have
expected intutively. They do however match pretty well the measurements by
the German guy, linked in this thread (sorry, cannot remember his name or
the links).

For those who want to experiment themselves, the latest update of the
FlexPDE "Mirror cooling model" can be dowloaded from:
http://home.hetnet.nl/~artm/atm/resource/resource.html

Thanks,
   Arjan te Marvelde

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