Re: [ATM] Amount of Under Correction??

[Mike Lockwood <melockwo@uiuc.edu>]

Peter,

Peter wrote:
> I'm working on a 16" f/4.5 2" thick Pyrex mirror (full thickness)
> and it currently has a very smooth figure, great edge so I'm
> getting close. I was told that fast thick mirrors should be under
> correction to compensate for the cool down throughout the night.
> I'm currently at 85% correction (just over 1/3 wave front).
> I wanted to ask it this is true that I should under correct the
> mirror and by how much?

Strictly speaking, 2" is not "full thickness".  16" / 6 = 2.67" is 
full-thickness.

Measure the mirror when it is slowly cooling (so that air currents do 
not prevent a reasonably accurate measurement), then measure when it 
when it is equilibrated.  Can you tell the difference?  I strongly 
doubt that you would get a 15% change in correction.  If there is a 
marked difference, check the blank for strain.

The cooling experiment is most illustrative when the mirror is 
spherical, as the change in shape is most readily observable.

Normally I would not undercorrect it at all, except for possibly a 
very small amount in the very outer zone.  That is generally where the 
most pronounced cooling effect would show because both the edge and 
the face of the mirror are radiating heat, and that can cause the 
outer part of the mirror to shrink more than the rest of the optical 
surface.

I'd only undercorrect the outer zone by a few percent at most, or a 
small fraction of a wavelength.

If you were to leave the mirror significantly undercorrected, your 
mirror will be flawed on the best nights when temperatures are falling 
slowly and the seeing is steady, and that would be a shame.  So, keep 
figuring.

	Mike Lockwood

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