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Optical Design For a 32
Inch, All-Spherical Relay
Cassegrain Telescope
Presented at Stellafane 2004
By: Scott Milligan
5/24/2017
Astroverted Optics
1
Motivation for this project
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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.
20” F/8 Mersenne:
primary field of view
20 “ F/8 Mersenne:
secondary field of view
What is a Relay
Cassegrain Telescope?
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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.
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Addition of relay optics solves the double
FOV vs. central obscuration problem
inherent with the Mersenne design.
Relay designs can use all-spherical optics.
An Example: 32” F/6 Relay
Cassegrain telescope
The “Classical-Cassegrain crunch”
Once EFL & BFD
are chosen,
obscuration ratio
and primary F/#
are closely (and
unfavorably)
coupled.
Cassegrain Obscuration ratio vs. primary F/#
as a function of system F/#: BFD is fixed
0.6
0.5
Obscuration ratio
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0.4
F/6
F/10
0.3
0.2
0.1
0
0
0.5
1
1.5
2
2.5
Primary f/#
3
3.5
4
4.5
5
An F/6 Cassegrain with a
44% central obscuration
An F/6 Cassegrain with a
25% central obscuration
Some advantages of relay
telescopes:
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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
Material to be removed during figuring from the vertex radius
5.00E+01
4.50E+01
Sag (waves @ 633 nm)
4.00E+01
3.50E+01
3.00E+01
32" F/3 Paraboloid
20" F/5 Paraboloid
10" F/5 Paraboloid
2.50E+01
2.00E+01
1.50E+01
1.00E+01
5.00E+00
0.00E+00
0
2
4
6
8
10
Aperture (inches)
12
14
16
18
And a few drawbacks…
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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.
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
Limitations of prior work
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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
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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
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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
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Spherical F/3
primary
Plano-CC ManginType secondary
Cemented doublet
field lens
Two singlet relay
lenses
Analysis: On-axis OPD Fans
Analysis: Spot Diagrams
Analysis: Lateral Color
Analysis: Field Curvature
Analysis: MTF curves
New design vs. several
other existing designs:
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A
A
A
A
32”
32”
32”
32”
F/6 Ritchey-Chretien
F/6 Classical Cassegrain
F/6 Newtonian.
F/7.9 Sigler-type Relay
Analysis: MTF for Several
existing designs
R-C MTF
Newtonian MTF
Cassegrain MTF
Sigler Relay MTF
Avg. MTF @ 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
Design variations: field
flattener works at F/8.6
MTF with field Flattener
Design variations: folded
Nasmyth focus; primary is
F/2.4
Design variations: folded
“outrigger”
Conclusions
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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.