Re: [ATM] Thermal mirror deformation

["Jan van Gastel" <jhm.vangastel@wanadoo.nl>]

The 'German guy' is Wolfgang Rohr, at
http://www.astro-foren.de/showthread.php?t=6454 .
Never realized that a If a 0.08 degree difference in temperature between
back and front of a 16 inch mirror already gives a deformation of 130 nm,
about 1/4 wave (assuming your assumptions are right). Hopefully most of it
can be taken care of by refocussing.

Jan
http://home.wanadoo.nl/jhm.vangastel/Astronomy/


----- Original Message ----- 
From: "Arjan te Marvelde" <arjan.te.marvelde@hetnet.nl>
To: <atm@atmlist.net>
Sent: Monday, August 14, 2006 9:08 AM
Subject: [ATM] Thermal mirror deformation


> 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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> ATM mailing list http://www.atmlist.net/
>

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