Re: [ATM] 25" mirror possibilities

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

-----Original Message-----
From: dmikk2@netscape.net <dmikk2@netscape.net>
To: atm@atmlist.net <atm@atmlist.net>
Date: Wednesday, November 10, 2004 12:24 PM
Subject: [ATM] 25" mirror possibilities


>  Thanks to everyone for your helpful suggestions, info on testing and
offer refigure the mirror.
>
>   Regarding the cost & shipping, the winning bid was $865.  The shipping
charge to my area was $125.  However that only got it to the freight
warehouse, so I had to rent a truck, make a simple cart with wheels, etc. to
get it home.  The package weighed 453 lbs, since the mirror was mounted to a
large steel ring that was part of the original assembly.  I've managed to
carefully unpack it and separate it from the mounting, without damaging it,
so now it's simplified to just the mirror ( estimated at 180 lbs ) lying on
its back on the cart.  So in all it cost about $1200 so far.
>
>  I need to make a sturdy stand before I can set it up for testing.  I have
held a small led above the mirror at the end of a stick.  The ceiling is
about 9.5 ft above the mirror surface and with the led at that point, the
reflected image still appears somewhat larger in diameter than the mirror.
Consequently I assume that the focal length is greater than 9.5 ft.
According to what I've read, a Wright Schmidt design would have the
corrector plate near the focal plane of the primary.  The original
instrument had the corrector at about 12.5 ft from the primary.  So...that's
the basis for my guess of about f6 for the mirror.  At least I can be sure
that it is not really "fast" like f2.5.
>
>Thanks,
>Dennis
>

don't start modifying it . If you wanted a blank, you could get a 24" blank
for less than you paid for this glass . See here for example:
http://pages.infinit.net/asmprod/glass.html , a 24" pregenerated to F/5 for
$999 . That is a 75 lbs blank not a 180 lbs , same thermal expansion as
Pyrex .
Take the time to very very accurately test what you have . Measure its
figure, and hope to be able to use it as it is in a scope designed around
its exact parameters , if it has a high enough surface accuracy and
smoothness . Regrinding it is not worth the effort, because it's going to
cost you more than making a mirror from scratch and it'll be something
different than what you wanted . If you wanted to start from scratch, sell
it and buy a blank that's more appropriate for the scope you're going to
build . Don't let the blank dictate what you are going to build, since you
may end up not using the instrument (due to being too heavy to be portable,
or too long to cool down, or too long focal length, which would also cost
you extra to mount and drive) .

best regards,
matt tudor


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