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CUAS 16.25” Telescope
Design Choices
Mike Lockwood
Oct. 14, 2004
Why me?
• My motivation:
– Every club should have a good scope
permanently set up and equatorially mounted.
– Dilemma - clubs have a hard time getting good
large optics. I enjoy making them, and I am
working for free. Problem solved!
– This is the best way for me to help.
– My optical work gets noticed.
Cassegrain Optics
• Many types (Classical, D-K, R-C, etc.)
• Primary mirror (F/3.76, FL = 3.76*16.25 = 61.13”)
– Concave – sphere, ellipse, parabola, hyperbola
• Secondary mirror – size TBD
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Convex – sphere, ellipse, parabola, hyperbola.
25% to 35% the diameter of the primary.
Slows down the convergence of light from the primary
Directs it out the back of the scope, through primary.
• The resulting F/ratio is typically 3 to 5 times the
primary focal ratio, F/12 to F/20.
Optical Diagram
Light
Making the Optics
• Primary mirror
– Grind, polish, figure, remove central “plug”.
– Most of this work is done – fix figure.
• Secondary mirror (not started)
– Grind and polish mirror and tool to sphere with
proper radius of curvature (curve).
– Figure concave tool to proper hyperbola
– Match convex mirror shape EXACLY to tool
shape with interference testing.
Possible Designs – Dall-Kirkham
• Ellipsoidal primary (less correction than a
parabola), spherical secondary.
• Pros:
– Secondary is easier (but NOT easy) to make,
slightly smaller (due to higher F/ratio).
• Cons:
– F/ratio is generally higher (F/16+) due to
increased off-axis aberrations (coma), resulting
in narrower field of view.
– Primary is not useful for other designs.
Classical Cassegrain
• Parabolic primary mirror, hyperbolic
secondary mirror
• Pros:
– Wider field views due to lower possible F/ratio
(F/12 and up). Easier to use.
– Parabolic primary is useful for a Newtonian.
• Cons:
– Secondary is more difficult to make.
– Slightly larger secondary mirror
Dall-Kirkham or Classical Cass?
• The intended design was a classical Cassegrain,
but the optics were not good.
• For general use, I recommend a classical
Cass. due to wider fields of view, less coma.
– I volunteered to test and fix the optics of a classical
Cassegrain. (Making my own 12.5”, too.)
– I know the work involved. I am building a machine.
– I have not made a Cassegrain secondary (yet).
– Should I need it, I can get advice from an optician who
has made many.
Let’s Decide Officially:
Dall-Kirkham or Classical Cass.
• Discussion, Questions
• Voting
Choosing the F/ratio
• The tradeoff: Focal ratio
• Lower F/ratio
– Wider field views (wider field)
– Larger secondary (30%), slightly less contrast
• Higher F/ratio
– Narrower field
– Smaller secondary mirror (25%)
• Design uses 0.6” fully illuminated field
F/Ratio Recommendation
• Lower F/ratio
– F/12.5 for classical Cassegrain, 5” secondary
– F/15 for Dall-Kirkham, 4.25” secondary
• Why?
– This is a general use scope. We want it to be
easy to find objects. Wider fields are good.
– Planetary views will not be much different with
25% obstruction than with 30%. (In any event,
they will be MUCH better than they were!)
Let’s Decide the F/ratio
• Questions, Discussion
• Voting
Other Recommendations
• 1) With good optics, cooling fans in the
main scope tube are a necessity. We need
12V power.
– Solar panels / batteries?
• 2) Improve the declination axis bearings.
– Evaluate balance point of tube with new optics.
– Add new metal-on-teflon bearings with tension
adjustment. (Not hard to do, and cheap.)
Controversial Recommendations
• 3) Remove the refractor from the tube!
– Allows the main scope to balance and to be
made to move much more easily in declination.
– Evaluate the current 80 mm finder, possibly
upgrade.
• 4) Build a tripod, dob-style cradle for it
– Then it will be portable, easy to set up outside,
and will probably get lots of use.
– Use it for finding stuff, or while the big scope
and dome are cooling off via the new fans.
Closing
• Let’s avoid coating failure on the new dob!
• Think about future mount options in the
dome, and how to make it more useable.
• Other useful things – headlight shield and
toilet facilities.