(ATM) Re: SecondaryFab/Conic Section/Bill's Compound Scope

[btk@ix.netcom.com (Bill Marriott )]

You Wrote:

>A conic section is defined by Schwartzshild's constant and paraxial
>radius of curvature ONLY. F/ratio doesn't matter.

Hey Folks, 

Thanks for the many responses confirming what I should of more 
confidently known, 

>If you have good ray-trace program, you can "create" a certain conical 
>surface, and make it either concave or convex simply by changing the 
>sign of the radius. So, say you've optimized your Cassegrain with 
>certain conic constant and radius. Simply "remove" the primary, 
>enlarge the secondary, change the sign of the "radius" column, and 
>instead of a parallel beam, shine a pencil of rays originating from a 
>radius of curvature onto it. They will come back not into the point 
>(like in
>case of sphere), but with certain longitudinal aberration. Pick 4
>or 5 zones on the secondary, note down the longitudinal aberrations
>needed, and you have a set of figures to work with ! Same as if you
>are striving for a parabola, but instead y*y/R for each zone, use 
>ray-trace progam provided figures as desired values (they will be 
>something like K*y*y/R anyway, where K is a constant).

Believe it or not, this is starting to make sense!

>After you've finished with a concave test plate, use inteference 
>method to make convex (I know, it's slow but I don't know of better 
>method to>get a good secondary). If your hyperbola is not too strong, 
>you MIGHT>get away with a spherical test piece, and look for certain 
>interference pattern (like in Texerau), but that seems a bit too 
>"loose" to me.

The plan was to make a hyper concave test plate, checking newtonian 
fringes as I do with flat work here in the shop, going for 'straight 
bands' as we say.

>Local retouching on a 2" secondary is almost impossible to do without
>screwing up the smoothness (as all opticians at Meade and Celestron
>know very well !)    :-)


>Bratislav

Once again thanks for everyone's input, this WILL be an interesting
and educational project!

Bill Marriott
Forest Knolls, Ca. USA
btk@ix.netcom.com