Re: [ATM] 40" mirror

["matt" <mariusrf@bellsouth.net>]

-----Original Message-----
From: Ken Lowther <hermit@outofoptions.org>
To: atm@atmlist.net <atm@atmlist.net>
Date: Wednesday, December 22, 2004 8:48 PM
Subject: Re: [ATM] 40" mirror


>Quoting "Jerry B. Hillman" <JBHillman@ev1.net>:
>
>> Hi Jeff,
>>
>>> Also consider a thinner mirror.   Four inches of glass will almost never
>>> come to equilibrium.
>>
>> I thought that is why pyrex is the substrate of choice.  Low thermal
>> expansion co-effcient.
>>
>
>Heat retention is a seprate issue.  I think Mel once said something like a
one
>inch mirror will take an hour to cool, a 2 inch about 3 and a 3 inch mirror
>will take longer than the observing session.  That is why I suggested you
>search the archives for this in my first post.  I probably have the times
>wrong, but the idea I beleive to be an accurate reflection of his post.
>
>I think active cooling design may have improved since then but 4 inches
>is still
>one heck of a chunk of glass to cool.
>
>Hermit, holed up in Youngstown, Ohio


an interesting link is here:
http://medusa.as.arizona.edu/lbtwww/tech/ua9502/ua5.htm

it largely refers to the Large Binocular Telescope and its giant 8m primary
mirrors but it has lots of interesting numbers for us amateurs.
Here are a few of them:
- mirror must track ambient . A 1 deg C of temperature tracking error
introduces 0.3 to 0.5 arcsec seeing turbulence .
- the LBT mirror thermal time constant is 30 min !!! I had to read that 5
times and good thing I was sitting at the moment. My 16 inch mirror has more
than 30min time constant . The LBT mirror thermal time constant of 30 min
requirement comes from their typical ambient cooling rate of 0.25 deg C/hour
.
- the LBT mirror is cooled with air from the back, nozzles that blow air at
ambient temperature , and the local air flow (debit) is proportional with
mirror local thickness and delta T .

best regards,
matt tudor

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