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[ATM] Schief secondary

Hi William,

you can use a full diameter secondary since it is a sphere. Just mount 
it like you would the nominal size secondary. You only use the small 
part of its area that is specified by the design, typically about  55mm 
in diameter.

Having made a 4.25 inch Kutter Schiefspiegler myself, I would not make 
the design that calls for equal but opposite curvature of primary and 
secondary design anymore.

There are designs in existence which call for two different radii, and 
offer even somewhat better correction. I would be happy to share design 
data.
This way, one can avoid cutting the secondary from the tool. You simply 
make a separate secondary, and enjoy fantastic images.
I was successful making a 85mm diameter convex secondary for another 
telescope design from ordinary 1cm thick window glass.

Guntram Lampert
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