Re: [ATM] undercorrecting?

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Tony and Jan, you are in a perfect position to test this theory, since 
you're both mirror makers.  Simply put one of your mirrors some place 
10-20 degrees warmer than your testing area, and let the mirror warm 
up.  Then set it on the test stand and see what test results you can 
get after you let the heat currents die down for a few minutes.  You 
can test every 30 minutes until the mirror goes back to its normal figure.

I hope you guys try this and report the results.

Note - ideally the mirror would not have its back flat against a solid 
test stand, it would be spaced away from it a bit so the back of the 
mirror could cool.  (Texereau does this with three nails - see his 
book for a drawing.)  That's a closer approximation to a mirror cell 
with good airflow.  Insulating the back of the mirror would probably 
accentuate the change in correction due to temperature, I suspect, and 
it will make the mirror cool slower.

Even with currents and not-so-great readings, you can easily tell a 
Strehl of 0.66 from 0.99.  I guess I'm used to seeing the Strehl go 
from 0.85-0.9 to 0.97+, but that's typically with Pyrex.  A 0.66 to 
0.97+ swing might be possible with plate.

What I notice most with a cooling mirror is that the outer zones do 
overcorrect a bit, and the edge can appear slightly turned.  This is 
due to the cooling area of the edge of the mirror disk - the outermost 
part of the mirror shrinks faster than the more central parts of the 
mirror because there is more surface area to cool them.  In the star 
test, this is usually seen as a false turned edge that persists until 
the mirror gets closer to equilibrium.

I do not share in the opinion that leaving a mirror significantly 
undercorrected is a good thing, since the temperature difference will 
cause air currents around the mirror and degrade the image anyway. 
However, if the outer zone is undercorrected by just a touch (1/20th 
wave or less), this may help counteract the "false TDE".

Some have also promoted the idea of insulating the edge of the mirror 
to reduce its cooling effect.  I've never tried it.  Maybe someone 
would like to and report back?

Mike Lockwood
http://bi-staff.beckman.uiuc.edu/~melockwo/index.html

tony gondola wrote:
> I'm thinking more of what I've seen with premium mirrors such as those by 
> Zambuto and others. These mirrors are 100% corrected with strehl ratios 
> typically of 99%. The actual performance of these optics in the field 
> doesn't seem to support the theory. I also don't think that it's true that 
> cheaper commercial mirrors are typically undercorrected and done so for the 
> thermal theory. The mirrors I've tested have been all over the place with no 
> clear trend other then not being very good. I don't think that the controls 
> are tight enough to consistently hit any number from most of those sources.


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