ATM A pair of Rumaks

[Aplanatic@aol.com]

The following contains the optical prescriptions
for a 200 millimeter aperture, f/10 and f/15 Rumak.
I include both a conventional optical description
and a TRACE file for each design.  All dimensions
are in millimeters.

The f/10 "Rumak":

Surface 1  RoC= -338.552 Glass=BK7
Distance 1 to 2 = 18.414
Surface 2  RoC = -349.155
Distance 2 to 3 = 607.386
Surface 3  RoC = -1592.2 SC=0 Mirror
Distance 3 to 4 = 513.017
Surface 4  RoC = -970.378 SC=0 Mirror
Distance 4 to focus = 751.81
Focal Surface RoC = -500
Back Focal Length = 238.793

The TRACE file follows:

Object
625.8 200
Refraction
BK7 625.8 338.552
Refraction
Air 607.386 349.155
Mirror
0 1592.2 0 250
Mirror
513.017 970.378 0 200
SphericalFocus
-238.793 500
WAVELENGTH
0.7 0.54 0.42
ANGLE
0 0.002 0.003
FOV
0.1

Notes:  Notice that in this design the secondary
 mirror has been moved away from the corrector.
 Thus, supporting the secondary from the corrector
 will require a short stock.  The axial distance from
 the corrector to the secondary is 94.369 mm.

The f/15 Rumak:

Surface 1  RoC= -334.8 Glass=BK7
Distance 1 to 2 = 21.03
Surface 2  RoC = -346.97
Distance 2 to 3 = 604.77
Surface 3  RoC = -1592.2 SC=0 Mirror
Distance 3 to 4 = 589
Surface 4  RoC = -600 SC=0 Mirror
Distance 4 to focus = 838.539
Focal Surface RoC = -500
Back Focal Length = 249.539

The TRACE file follows:

Object
625.8 200
Refraction
BK7 625.8 334.8
Refraction
Air 604.77 346.97
Mirror
0 1592.2 0 250
Mirror
589 600 0 200
SphericalFocus
-249.539 500
WAVELENGTH
0.7 0.54 0.42
ANGLE
0 0.002 0.003
FOV
0.1

In this design the axial distance from the corrector
to the secondary mirror surface is 15.77 mm.  The
above f/15 prescription is an optimized version of
Rutten and van Venrooij's design.

Additional notes:  In both cases above the on-axis spot sizes
are quite good, showing a normalized transverse aberration
that is less than about 0.5.  The fabrication tolerances for good
performance are more stringent than I have seen in any other
design.  One can vary the f/ratio from 15 to 10 by a number
of methods.  The method that I used in the above was quite 
simple:  Change the RoC of the secondary and move it towards
the primary in order to maintain a reasonable back focal length,
then optimize the corrector for this new system.  In practice, one
might wish to move the corrector inward with the secondary in
order to make the tube length shorter.  I have not tried this.  One
could also adjust the RoC of the primary.  I have not included the
sizes of the mirrors or other apertures in the system.  

Dave Rowe
Torrance, CA
Medium Format Astrophotography:
http://members.aol.com/aplanatic/photos/astro.html