Re: [ATM] undercorrecting?

["Dream - Telescopes & Accessories, Inc." <shane@dreamscopes.com>]

Tony & Jan,

If the mirror is truly equalized, how can the front surface be at a 
different temperature than ambient?  Then it is not equalized.

Bringing a mirror to thermal equilibrium is a far bigger issue than whether 
the aluminized surface radiates off this or that.  Even without aluminizing 
the sky surface will radiate off, when NOT thermally equalized, more than 
the back of the glass.  Until the glass reaches thermal equilibrium, 
quality is greatly diminished.  Concentrating on getting the glass to 
equilibrium is THE most important issue, after collimation of course.  Once 
the glass is at thermal equilibrium other games like horizontal air flow 
across the surface or insulating the optic's edge are not needed.  Get the 
glass to ambient and help it follow ambient.

You can find reports on the web from large observatories that deal with 
these issues: front surface sky radiation of the main optic.

Large scopes have the same issues but it isn't nearly as feasible to get 
the glass to ambient because the mass of the glass is so much larger.  They 
have used all sorts of techniques to control the optic's temperature.  Some 
cool the inside of the observatory during the day.  A few hours, in 
general, prior to a planned observatory opening, they start reducing the 
inside temp so that when they open up their inside and outside temps and 
therefore the optic are the same.  Once they open up however they have to 
use other methods to try to help the glass follow the constantly changing 
ambient temp.

Another technique is air jets on the back of the primary mirror.  I see 
these used most often on cellular mirrors but it works for plano-backed 
mirrors as well, just not nearly as efficiently as cellular.  These can be 
used during both day and night to help control optic temps but are 
especially useful after the observatory is open.

If a large optic can't be made to follow ambient temperatures, this is when 
they start worrying about sky radiation and other issues.  But for small 
optics, just concentrate on getting lots of air flow to the main optic.  A 
large box fan should not be out of anyone's budget.  Power for a visual 
observer that takes his setup to a remote location might be though.

The best solution to fight all of this is to concentrate on getting the 
optic to thermal equilibrium.  If the main optic is buried inside a 
housing/structure, it will not be able to cool very well.  This is why my 
primary mirror is so exposed on my telescopes:
http://www.dreamscopes.com/pages/projects-04/newt-astro-16-shroud.htm

I never put the bottom shroud all the way down, covering the sides of the 
primary mirror.  Inside the observatory is a high volume fan moving large 
quantities of air throughout the entire observatory.  I normally put a 
second fan aimed at the primary for additional and concentrated air 
flow.  While doing testing and forced to be inside the observatory, I aim 
the second fan so that it hits the primary first, then is aimed in my 
direction.  If the opposite was used, a fan near me and the computer(s) was 
aiming at the primary, it would be blowing warmer air at the 
primary.  Because the primary mirror is so exposed, air is free to move 
past the main optic and helps the glass follow ambient.  Encasing an optic 
to keep it clean will do one thing.  Keep your optic clean.  It will 
degrade everything else though, much more than some dust and dirt are 
degrading the image.  If you have a good optic, thermal issues are 
everything.  As well as collimation (star collimation).

Thank you.

Sincerely,
Shane Santi - President
Dream - Telescopes & Accessories, Inc.
http://www.dreamscopes.com
610 - 365 - 2833



At 01:02 PM 8/3/2006, you wrote:
>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/
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