Re: [ATM] undercorrecting?

[Mike Lockwood <melockwo@uiuc.edu>]

Arjan,

Arjan te Marvelde wrote:
> Dunno, could be anything, really.  Maybe it's stress in the glass? I just
> heard from Jan van Gastel that this specific mirror has been (re-)figured a
> couple of times.

Maybe.  Lots of possible causes.  Wonder what eyepiece he's using?  :)

> You will see that polished Al is 0.04-0.06, and CVD Al with SiO overcoat is
> set at 0.04. For the various kinds of glass you can take 0.95. Polishing
> does not change that much.

OK.  0.04 it is.

> I did not take radiation from the back into account, since the ground
> temperature will be approximatly equal to the mirror temp. If this is not
> the case, the high emissivity of bare glass will quickly make this a factor
> (either heating or cooling).

Sorry, I didn't mean to say emissivity.  I meant to question the 
difference between a polished mirror front and a ground back when 
subjected to convective cooling.  It might account for a 0.014 degree 
difference or larger.

> Good point, if you have high-school physics/math level I think you should be
> able to work with the FlexPDE model. You just install the program, obtain a
> student license, and load/run the model from my website. Changes can be
> easily made in the model in text format, e.g. by providing a profile for the
> heat transfer parameter k, instead of the constant it currently is. For the
> nifty things there's a help file...

Wish I had the time, but I will have to let someone else experiment.

	Mike L.

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