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Optical Design For a 32 Inch, All-Spherical Relay Cassegrain Telescope. Presented at Stellafane 2004 By: Scott Milligan. 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!

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optical design for a 32 inch all spherical relay cassegrain telescope

Optical Design For a 32 Inch, All-Spherical Relay Cassegrain Telescope

Presented at Stellafane 2004

By: Scott Milligan

Astroverted Optics

motivation for this project
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
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
The “Classical-Cassegrain crunch”
  • Once EFL & BFD are chosen, obscuration ratio and primary F/# are closely (and unfavorably) coupled.
some advantages of relay telescopes
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.
and a few drawbacks
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
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
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
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
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
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
Analysis: MTF for Several existing designs

R-C MTF

Cassegrain MTF

Newtonian MTF

Sigler Relay MTF

conclusions
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.