Optical Design For a 32 Inch, All-Spherical Relay Cassegrain Telescope Presented at Stellafane 2004 By: Scott Milligan Astroverted Optics
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.
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.
The “Classical-Cassegrain crunch” • Once EFL & BFD are chosen, obscuration ratio and primary F/# are closely (and unfavorably) coupled.
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.
Material to be removed when figuring: three different Paraboloids compared
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.
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.
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 420-900 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.
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 400-900 nm. • Improved Off-axis imagery (wrt prior art). • Accessible focal plane.
Description of Layout • Spherical F/3 primary • Plano-CC Mangin-Type secondary • Cemented doublet field lens • Two singlet relay lenses
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
Analysis: MTF for Several existing designs R-C MTF Cassegrain MTF Newtonian MTF Sigler Relay MTF
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.