From: "Mike and Sara" 
To: 
Reply-To: "Mike and Sara" 


Hello!  Before I start to fingure my primary 12.5 inch F3 mirror for my
Dall-Kirkham, I still have time to change the design to a classical
cassegrain.  I ground the spherical secondary and it is awaiting
installation, and I was going to figure the primary via Foucault then star
testing.

    However, I can purchase a plano-convex lens from Melles-Griot, 600mm FL,
95mm diameter, BK7 glass, with center thickness of 6.6mm abd edge thickness
of 3mm with a design index of 1.5187 with a design wavelength of 546.1.  I
can figure the primary to a paraboloid with foucault/star test, and then
polish this lens to the correct hyperbola shape.

    Given the specs on the lens, can anyone tell me what the foucault
numbers would be to give a hyperbola on through the back testing? 
According to my design program, for a spacing of 813.75mm between the
primary and secondary, with focus 1050mm from the secondary, the conic
constant of the secondary should be -1.65.

    Finally, I know how tough it was to polish and grind my 90mm blank
spherical with a ROC of 306mm which I evaluated by intereference fringe
testing with a polishing a mating concave sphere which I evaluated by
foucault testing.  Am I biting off more than I can chew to try and polish
this lens to a hyperbola using through the back testing and the system
testing with the star test?  Thanks and best regards.

Mike

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