Re: [ATM] Thermal mirror deformation

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

Scott & group,

What Scott proposes has already been done on a multi-meter telescope.  Air 
ejectors/jet ejectors at the edge of the primary mirror, around the entire 
circumference, shoot a concentrated and low profile stream of air to the 
center of the mirror.  Air intakes are positioned near the central hub to 
eject that air and create an ideal laminar air flow pattern.

I can't find the paper that describes the project above but you can google 
air ejectors or jet ejectors for primary mirrors.

MMT and LBT are two large projects that use just air ejectors, in a 
different way.  Both of these large telescopes use them to cool the back of 
the cellular mirror(s).  LBT uses 252 for each of it's two 8.4m mirrors.

You can see an illustration of one of MMT's air ejectors for the 6.5m 
primary mirror on page four of the following document:
http://www.mmto.org/MMTpapers/spie/spie5489-127.pdf#search=%22MMT%20exhaust%20ejectors%22

Gemini, a 8.1m telescope, uses a completely different method but one that 
other observatories are use in combination with things like air ejectors on 
the back of the primary mirror.  You can see Gemini's observatory in a 
fully open position here:
http://www.gemini.edu/index.php?option=content&task=view&id=9&Itemid=2

One or more anemometers are positioned near the primary mirror's outer 
structure.  These feed information to a software program that controls the 
louvers.  Wind across the primary mirror can therefore be controlled to a 
set point by controlling the louvers' position.

What all of these have in common is large amounts of air directed at the 
primary.  If you have the means to throw large amounts of air at the 
mirror, it will eliminate thermal issues like dew and glass that isn't 
equalized.

Thank you.

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



At 12:25 PM 8/30/2006, you wrote:
>Someday, I'll get around to trying this idea, which has been kicking 
>around in the back of my head for years, but for now, I'll just ask the 
>group; Has anyone tried using a mirror with a central perforation, mounted 
>on a central hub that also functions as a plenum for sucking a continuous 
>stream of air from the region in front of the  mirror surface and 
>exhausting the air out the back? The air might enter the plenum via a 
>multitude of radially oriented holes, located a small distance in front of 
>the optical surface.
>
>Scott Milligan
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