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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 – – – – 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.