Re: [ATM] undercorrecting?

[Mark Cowan <toolontop@yahoo.com>]

Hi Mike and all:

I'm taking this out of context of its original
discussion, but I came across it and had some
interesting information to convey that originated in
discussions with Steve Kennedy (who makes very large,
fast mirrors - 28" f/3.66 - Tom O. owns a 24" f/3.75
that I checked out at OSP and it was basically
flawless).

These are 2" or 2-1/8" thick and will not easily reach
complete thermal equilibrium, so he favors a small
amount of undercorrection right /at/ the edge, and an
even smaller amount of overcorrection right /inside/
the edge,  IIRC.  He prefers to remain on the side of
undercorrection for various (good) reasons as well.

Steve does all final figuring with star test, as do at
least a few other people (well, not me!).  He will
test into the night during cooling and again in the
early morning, and compares this to Couder mask on the
bench, but in the case of a discrepency he relies on
the the star test.

What he's noticed is that when he's carefully tested
mirrors after coating (with a standard overcoated Al
from Spectrum) the mirror will show stronger
correction in the same star test enviroment after
coating as to before.  I doubt there's any significant
stress on the glass to account for this, so I
suggested to him that the change must be due to the
alteration of radiation characteristics off the front
surface.

Best,
Mark Cowan

--- Mike Lockwood <melockwo@uiuc.edu> wrote:

[snip]
> What seems to be overlooked here is how the front
> and back of the 
> mirror are prepared.  The front is polished and
> coated with aluminum 
> and silicon dioxide (what's the effective emissivity
> of the 
> combination?), while the back is probably not.  What
> difference does 
> that make for the emissivity of glass?  Is it larger
> than the effect 
> of radiative cooling on the mirror?
[snip]

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