Re: ATM my 8.5" cass project

["Bob May" <bobmay@nethere.com>]

Tony, you really need to sit down and figure out what the scope is that you
are doing.  A RC type cass is two hyperbolas and provides probably the best
design for a lot of work.  This is a quick design from TDESIGN:
   F 2.6 RC
           Clear Aperture                                   212
           Focal Length                                   2000.0
           Minimum Secondary Diameter          49.88
           Geometric Focal Ratio                      9.43
           Photographic Focal Ratio                  9.71
           Primary Radius                              -1100.000
           Primary Aspheric Deformation         -1.046538
           Secondary Radius                          -356.998
           Secondary Aspheric Deformation     -3.611770
           Primary to Secondary Distance        -420.59
           Secondary to Focal Plane Distance   470.59
           Primary to Focal Plane Distance        50.00
I did the thing in sillymeters which shouldn't cause any real problems for
you.
Note that the Aspheric for the primary is just over a parabola but the
secondary is about 3.6x the parabolic.  The distance between the surfaces is
about 16" and the focal plane is 2" behind the primary surface.  Here's a
second one for 4" instead:
        Clear Aperture                                   212
        Focal Length                                     2000.0
        Minimum Secondary Diameter             54.04
        Geometric Focal Ratio                       9.43
        Photographic Focal Ratio                   9.76
        Primary Radius                               -1100.000
        Primary Aspheric Deformation           -1.051743
        Secondary Radius                           -386.748
        Secondary Aspheric Deformation      -3.625429
        Primary to Secondary Distance        -409.80
        Secondary to Focal Plane Distance   509.80
        Primary to Focal Plane Distance       100.00

Neither of these have been raytraced so you may want to play with them in a
raytracer to get a more precise number.  Note that both of them require
about a 2" diameter secondary so a 1/4" thick piece of glass would be
adequate for the job although you are going to want to make it a bit larger
in diameter for a start to insure that you don't have a turned edge.  Have
fun making the secondary.  You might want to stop by my website and check
out how to test a secondary with the Hindle Test.

Bob May
My new web space address is http://webu.wigloo.com/bobmay/ or
http://nav.to/bobmay
and my new email address is bobmay@nethere.com
Bob May