Re: [ATM] 40" mirror

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

Jerry,

You are right in one aspect, you won't be able to lift the scope.
However, reducing primary weight matters a lot . For the extra 100 lbs in
the primary, you will need a stronger and heavier cell, then a stronger
scope structure, starting with mirror box. Then a heavier scope front to
balance the heavier mirror , cell and mirror box. You will add 200 lbs to
the scope probably for the heavier mirror.
Then for the extra scope weight the drive system if any will have to be
stronger/heavier duty . It all leads to more engineering complications. The
extra strength costs money to  engineer .
The main problem is not these complications however.

The main issue will be the mirror and scope thermal time constant . A giant
scope like the Large Binocular Telescope with its twin 8m mirrors has a
mirror thermal time constant of 30-40 minutes. This is due to the mirrors
being very thin , having a cellular structure and no more than 1 inch thick
mirror face  . This very short time constant was required due to the average
location cooling rate each night of 0.25 deg/hour.
I'm sure your scope will end up in a location that is not as good as the
location for the Large Binocular Telescope. You will have worse thermal
changes over night . If your mirror and scope is a behemoth with 4 inches of
glass it will never track ambient. Some previous post , don't recall if in
this thread but it was recent , mentioned that Mel Bartels had a rule of
thumb that a 4 inch thick mirror never reaches thermal balance .

I've seen quoted a number of 0.3 to 0.6 arcsec seeing degradation due to 1
degree of temperature imbalance between mirror front and ambient . If your
mirror is always 2-4 degrees above or below ambient because it can't track
the 1-2 deg per hour temp changes you may experience, your scope will always
work with its own internal seeing degraded to no better than 0.6 to 1.2
arcsec . Add to that under even the best atmospheric conditions another 0.5
to 2 arcsec and you will NEVER have any type diffraction limited resolution
from your scope. It will always be limited by scope seeing due to cool down
too slow .

Better take the time now to engineer a solution to the problem than ignore
it and later abandon the scope as being a white elephant . Too many
beautiful projects end up like that due to our haste to proceed with
building before designing . Even if we're amateurs we can still do at least
some back of the envelope calculations . Why waste years of your life
building something half baked or putting money into unnecessary mechanical
bulkness when you could try to reduce it a little and target cooling time
and thermal management instead. Even if you just got some professional help
with the mirror for the money you'd otherwise put into the stronger/heavier
structure, you'd still end up with a better scope for not more money .

best regards,
matt tudor

-----Original Message-----
From: Jerry B. Hillman <JBHillman@ev1.net>
To: Jeff Anderson-Lee <jonah@eecs.berkeley.edu>
Cc: The Amateur Telescope Makers List <atm@atmlist.net>; Jay Kirkland
<jaykirk2@compusmart.ab.ca>
Date: Thursday, December 23, 2004 5:22 PM
Subject: Re: [ATM] 40" mirror


>Hi Jeff,
>Shaving an inch off would save 100 pounds.  It would also make it 1/13
>ratio. That is way better than the 1/22 ratio that my 16.5" mirror was (at
>the edge) but still dealing with the difference between 300 and 400 isn't
>going to matter that much.  I can't lift it regardless so it is still an
>engineering problem.
>
>Clear skies, Jerry
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/

_______________________________________________
ATM mailing list http://www.atmlist.net/