Re: [ATM] undercorrecting?

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

Mike,

That's an interesting experiment as, as you say, not difficult to perform.
But the question was not whether a mirror deforms or not during cooling
time, but if a mirror, once at ambient temperarure, deforms that much
because of cooling of the aluminiumized surface by radiation into the
kosmos, while the back of the mirror stays at ambient temperature. I can't
imagine that radiation has such a large effect, but some do. I wonder if
there are other experiments then the one in the link in my former posting.

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


----- Original Message ----- 
From: "Mike Lockwood" <melockwo@uiuc.edu>
To: <atm@atmlist.net>
Sent: Thursday, August 03, 2006 5:31 PM
Subject: Re: [ATM] undercorrecting?


> 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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>

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