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.
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.
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.