From: "Kreig McBride" 
To: "Bob Walker" 
Cc: 
Reply-To: "Kreig McBride" 


I cant imagine glass for an 11" scope costing $20,000.  Flint and
crowns in such sizes are less than a grand.

You can probably cast the plastic as accurate as you need with no polishing
or figuing, however trying to get a homogeneous pour would be imposssible.
Not sure if there is anything close to the refractive indices needed to
make such a scope.   Glass is a lot easier and more predictable.

Kreig


-----Original Message-----
From: Bob Walker 
To: ATM Group 
Date: Thursday, June 26, 2003 11:09 PM
Subject: ATM Large refractors


>
>Here's a question for you wizards:  How about making a large refractor out
>of plastic?  One of the stumbling blocks to making refractors is the
>astronomical (sorry) cost of large pieces of optical glass.  What about
>using optical plastic of the types used for  making eyeglasses?  Is it
>available in liquid form like the familiar polyester resin/hardener
>combination used with fiberglass?  If so, a large lens blank could be cast
>easily.  Could it be ground and polished by the same methods/materials used
>on glass?  (Much more quickly, one would think!)  Another barrier to large
>lenses is that they sag under their own weight.  Plastic is much lighter
>than glass, though also less stiff.  Would its stiffness/weight ratio be
>better or worse than glass?   Are said optical plastics available with
>suitable refraction / dispersion characteristics to allow design of an
>achromat?  What got me thinking about all this was reading about an 11"
>refractor being made by amateurs the glass for which cost $20,000!  Less
>than some of us spend for a car, but still prohibitive for most.  Think of
>it - the largest refractor in the WORLD is one lousy meter.  Some ATMs have
>made mirrors bigger than that.  What a coup if one could make a bigger
>refractor!
>
> *  Best regards, Bob
>*
>         *           *   *
>
>            *         *
>
>

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