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 Be the first pioneers to continue the Astronomy Discussions at our new Astronomy meeting place...The Space and Astronomy Agora Minimum Thickness Forum List | Follow Ups | Post Message | Back to Thread Topics | In Response ToPosted by Robert May on October 2, 2004 20:54:06 UTC

19mm will be enough to do a 250mm scope.
The problem you run into is the depth of the center of the curve compared to the edge thickness on a mirror when you get done.
Try to keep the center thickness no less than 2/3 of the thickness of the edge. In other words, a center that leaves .5" at the center and is .75" at the edge is about as much as you should go for a mirror that will work. The bigger the diameter of the scope, the easier it is to get a large depth at the center of the mirror so check to see what it is going to be.
FWIW, the Sag formula is S = R - (R^2 - r^2)^.5
Where:
R = Radius of curvature to be done on the surface
r = Radius of the circle you are measuring the sag at.
S = the Sag of the curve.
^2 is squared while ^.5 is the square root.