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CUAS 16.25” Telescope Design Choices

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.

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CUAS 16.25” Telescope Design Choices

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  1. CUAS 16.25” Telescope Design Choices Mike Lockwood Oct. 14, 2004

  2. 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.

  3. 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 • 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.

  4. Optical Diagram Light

  5. 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.

  6. 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.

  7. 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

  8. 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.

  9. Let’s Decide Officially:Dall-Kirkham or Classical Cass. • Discussion, Questions • Voting

  10. 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

  11. 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!)

  12. Let’s Decide the F/ratio • Questions, Discussion • Voting

  13. 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.)

  14. 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.

  15. 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.

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