[Raytrace] A real world application

[Peter Chen]

	If anyone is interested, I have a real world problem for which I'd 
appreciate community input.

	In the current work I'm doing we are producing a dozen or so lightweight 
composite mirrors.  These are all 20" diameter, f/1.9, spherical 
mirrors.  (Some of you may have seen the one I showed at Stellafane last 
year).  I would like to build a couple of simple minded telescopes out of 
these mirrors for making portable scopes to observe point sources (= stars).

          The primary being spherical, the best that can be done in a two 
mirror system is to make a inverse Dall-Kirkham (or Pressman-Carmichael, 
Rutten's terminology).  The secondary is a convex spheroid.  This is a zero 
field system.  The image is a spot only on the optical axis.  I've added a 
diagonal flat for finding a target in the field.

	The system requirements are as follows:
Primary - 20 in diameter, 76 in. ROC, sphere
Secondary - 4 in diameter (to reduce fabrication cost, actually any size up 
to 6 inch is ok)
Diagonal flat -  to be located 2 inch in front of primary, for ease of mounting
Focus  - 13 in. above optical axis (to accommodate focuser)

           Running these numbers through Rutten and van Venrooij's TDesign 
program, I got a set of preliminary numbers.  I then put them into Zemax 
and OSLO to check out the design and try some optimization.   So far the 
best I can get is something like the following: (OSLO format)


*LENS DATA
20 inch inverse dall-kirkham
  SRF      RADIUS      THICKNESS   APERTURE RADIUS       GLASS  SPE  NOTE
  OBJ       --        1.0000e+20    1.0000e+17             AIR

   1        --        772.160000     50.000000 X           AIR

  AST  -1.9304e+03   -772.160000    254.000000 AKX     REFLECT

   3        --        721.360000     50.800000         REFLECT   *

   4        --            --            --                 AIR   *

   5        --            --         22.779000         REFLECT

   6        --       -330.200000        --                 AIR   *

  IMS       --            --          6.000000

*CONIC AND POLYNOMIAL ASPHERIC DATA
  SRF        CC          AD          AE          AF          AG
   3       8.959129      --          --          --          --

*TILT/DECENTER DATA
   4     DT    1         DCX       --       DCY       --       DCZ       --
                         TLA   -45.000000   TLB       --       TLC       --
   6     DT    1         DCX       --       DCY       --       DCZ       --
                         TLA   -45.000000   TLB       --       TLC       --

The problems are:
    1.  It doesn't focus to a spot, even on axis.   The secondary roc 
doesn't seem quite right.
     2.   I have no idea how difficult it would be to make a spheroid with 
a conic constant of 8.959129.  Is that realistic?  How does one test such a 
piece?   (note:  The piece will be concave.  We replicate with it and get 
the convex part).

:  Does anyone have any suggestions?

    Happy Holidays, all.

Regards,
P.C. Chen