Re: ATM Removing material from a mirror blank for less weight

[Bob Goff <goffaxe@azstarnet.com>]

Grind the profile of the backside to a single arch parabola following the
formula: 

In my case it is a 16" F/3.2 Cass primary with a C.T. of 2.5 and a center
hole of 3" surrounded by a 1/2" of flat glass for mounting.
It's formula is X=2.667Y^2 + 2.0  .
Your formula would be slightly different because of the different ROC and C.T.
This would give nearly a tangential fit ot the front to back edge and has
minimum weight to stiffness ratio.
I remember Dillworth's design and instrument quite well even though it has
been 25 or so years since I last saw Don. I hope thaat he is well.
The piece of Pyrex that he used to make his primary from was one of the
first "large ones" made at Corning in the late 20s. I sold it to him way
back then. .

Bob



At 09:02 PM 08/23/2001 EDT, you wrote:
>
>Hi all,
>
>I am still in the process of trying to make a Dilworth type scope.  Keeping 
>in mind that the secondary mount is supported by the hole in the full 
>thickness primary mirror.  The saving grace is that the blank is ground on 
>the back side in the shape of a cone, removing much material from the back 
>sides outer edge yet the center portion it is still about 3" thick and the 
>outer edge is now about 1" thick. (similar to a Meade/Celestron SCT mirror)  
>In order to remove even more unnecessary weight, is it possible to sand
blast 
>away excess glass, again similar to the way the SCT are cast with the ribs
on 
>the back side.  I have access to a heavy duty sand blaster and can get the 
>protective material that they use for making sand blasted red wood signs.  ( 
>the thick rubbery material w/sticky back is placed on the wood and cut
with a 
>sharp knife, the sections to be blasted are pealed off, so the wood shows 
>through while the rubbery stuff protects the letters or design(s) while the 
>sand eats away the unprotected wood... so I was thinking would this work to 
>make ribs on the back side of a mirror.  How fast would the sand cut the 
>glass?  how accurate would the depth of the cuts have to be from the area 
>between one rib to the next area? Anyone ever try this type of thing?
>
>I know many have suggested using thinner glass, but I wanted to try to
make a 
>scope similar to Dilworths utilizing the beefy center hole for mounting 
>purposes, it's a life long goal thing, been wanting to do this for 20 some 
>years, just need to develop the skills!
>
>David
>
>
>
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