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

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

• Presented at Stellafane 2004
• By Scott Milligan

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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.

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20 F/8 Mersenne primary field of view
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20 F/8 Mersenne secondary field of view
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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.

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An Example 32 F/6 Relay Cassegrain telescope
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The Classical-Cassegrain crunch

• Once EFL BFD are chosen, obscuration ratio and
  primary F/ are closely (and unfavorably) coupled.

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An F/6 Cassegrain with a 44 central obscuration
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An F/6 Cassegrain with a 25 central obscuration
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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.

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Material to be removed when figuring three
different Paraboloids compared
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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.

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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
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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.

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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.

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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.

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Description of Layout

• Spherical F/3 primary
• Plano-CC Mangin-Type secondary
• Cemented doublet field lens
• Two singlet relay lenses

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Analysis On-axis OPD Fans
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Analysis Spot Diagrams
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Analysis Lateral Color
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Analysis Field Curvature
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Analysis MTF curves
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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

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Analysis MTF for Several existing designs
R-C MTF
Cassegrain MTF
Newtonian MTF
Sigler Relay MTF
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Avg. MTF _at_ 20 cy/mm for 5 Designs
Design On-axis 6 mm 12.3 mm
R-C 0.83 0.83 0.78
Milligan 0.87 0.81 0.60
Newt. 0.89 0.73 0.41
Cass. 0.83 0.68 0.35
Sigler 0.84 0.45 0.07
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Design variations field flattener works at F/8.6
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MTF with field Flattener
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Design variations folded Nasmyth focus primary
is F/2.4
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Design variations folded outrigger
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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.