1. Optical Design For a 32 Inch, All-Spherical Relay Cassegrain Telescope
Presented at Stellafane 2004
By: Scott Milligan
11/23/2018
Astroverted Optics

2. Motivation for this project
20” Mersenne telescope exhibited by Clyde Bone at 199X Stellafane.
Seated observing position in a large aperture instrument working at F/5!
But: Mersenne suffers from double field of view, and/or excessive central obstruction. Also requires fabrication of 2 parabolic mirrors.

3. 20” F/8 Mersenne: primary field of view

4. 20 “ F/8 Mersenne: secondary field of view

5. What is a Relay Cassegrain Telescope?
A telescope offering:
A reflective “Front end”
A relatively compact, folded optical path.
Relay optics re-image an intermediate image of the scene to an accessible location.
Addition of relay optics solves the double FOV vs. central obscuration problem inherent with the Mersenne design.
Relay designs can use all-spherical optics.

6. An Example: 32” F/6 Relay Cassegrain telescope

7. The “Classical-Cassegrain crunch”
Once EFL & BFD are chosen, obscuration ratio and primary F/# are closely (and unfavorably) coupled.

8. An F/6 Cassegrain with a 44% central obscuration

9. An F/6 Cassegrain with a 25% central obscuration

10. Some advantages of relay telescopes:
Can achieve excellent imaging on-axis over a wide range of F/# (F/4 – F/20).
Accessible image location without requiring large central obstruction.
Fully baffled without vignetting an extended field of view.
All-spherical designs eliminate requirement to fabricate & test aspheric surfaces.

11. Material to be removed when figuring: three different Paraboloids compared

12. And a few drawbacks…
Off-axis imagery is (typically) limited by field curvature associated with the use of positive focal length relay lens optics.
Added complexity of design requires careful analysis of, and attention to fabrication and alignment tolerances.
Collimation tolerances can be tighter than for equivalent, traditional Cassegrain.
Spectral bandwidth may be limited in comparison with all-reflective designs.

13. Historical Development of Relay Telescopes
Inventor
Year
Comments
H. Dall, B.Cox
1947, 1962
D.K.”twitcher” with corrected relay
R. Buchroeder
197X
All-spherical, XX elements
D. Dilworth
1976
All-spherical, 16” Built & shown at Stellafane
R. Sigler
1982
All-Spherical, simplest possible construction

14. Limitations of prior work
Dall & Cox designs difficult to correct for secondary spectrum w/o using expensive glasses.
Dilworth & Sigler designs offer no control over off-axis astigmatism.
These limitations motivated a search for an improved relay design.

15. Milligan Relay Cassegrain
Uses the Dilworth & Sigler designs as a starting point.
Improves correction for secondary spectrum and spherochromatism to achieve better than Diff. Ltd. Imaging on-axis over an extended spectral range nm.
Improves off-axis imagery by balancing field curvature with over-corrected astigmatism.
Creates a near telecentric exit pupil for ideal matching with modern wide field eyepieces.

16. Primary Design Goals All-Spherical optics 32” aperture, working at F/6
Central obstruction ≤ 25%
Use no exotic, “un-obtanium” glasses.
Illuminate a 46 mm image circle without vignetting.
Excellent on-axis imagery over a wide spectral band nm.
Improved Off-axis imagery (wrt prior art).
Accessible focal plane.

17. Description of Layout Spherical F/3 primary
Plano-CC Mangin-Type secondary
Cemented doublet field lens
Two singlet relay lenses

18. Analysis: On-axis OPD Fans

19. Analysis: Spot Diagrams

20. Analysis: Lateral Color

21. Analysis: Field Curvature

22. Analysis: MTF curves

23. New design vs. several other existing designs:
A 32” F/6 Ritchey-Chretien
A 32” F/6 Classical Cassegrain
A 32” F/6 Newtonian.
A 32” F/7.9 Sigler-type Relay

24. Analysis: MTF for Several existing designs
R-C MTF
Cassegrain MTF
Newtonian MTF
Sigler Relay MTF

25. Avg. MTF @ 20 cy/mm for 5 Designs
On-axis
6 mm
12.3 mm
R-C
0.83
0.78
Milligan
0.87
0.81
0.60
Newt.
0.89
0.73
0.41
Cass.
0.68
0.35
Sigler
0.84
0.45
0.07

26. Design variations: field flattener works at F/8.6

27. MTF with field Flattener

28. Design variations: folded Nasmyth focus; primary is F/2.4

29. Design variations: folded “outrigger”

30. Conclusions
Relay Cassegrain designs can achieve accessible eyepiece locations in large aperture scopes without requiring the user to tolerate a double FOV or a large central obstruction.
A new all-spherical relay Cassegrain design is presented that substantially improves upon the imaging performance of previously published, similar designs.