| The 20.1 inch Telescope |
Here is a picture of the mirror being polished. Polishing petals are being used at this time. They are directly attached to the 15" tile grinding tool. When this picture was taken, the mirror had been polished for a total of a slightly over three hours. The center is completely polished out and the edge is about 50% polished. This is real grunt work - but very invigorating. I'm glad I didn't build a grinding/polishing machine or I would have missed out on all of the fun!
In the following image, the mirror has been polished for about 5-6 hours with the pads. It is now completely polished out, no significant pits remain anywhere on the glass. The focal length is 110.25 inches, making the optic F5.5. Preliminary knife-edge testing shows the surface to be somewhat hyperbolic, about 20% over-corrected and with a slightly rolled edge. Otherwise, the surface looks smooth and uniform - not bad coming directly from polishing pads!
I am about to commence with figuring. I am in the process of constructing a knife-edge/ caustic test rig with dial indicators on both X and Y axes.
I have a 15 inch, a 10 inch, and 6 inch tools ready to for figuring laps to be cast with Gugolz 64 pitch. This is my first attempt at figuring with sub-diameter tools, and I hope to follow the methodology outlined by Robert Kester in Krieg and Berry's new book, "The Dobsonian Telescope".
Here is the secondary cage assy. Rings are 15 ply Baltic Birch. Edges have been rounded with 3/16" radius router bit. Focuser board has pockets cut with milling machine for lightning. Center pocket size matches base of the focuser (crayford - style focuser will be custom made).
I haven't decided if I want to build my own spider-diagonal holder, or cheat and buy a ProtoStar unit. Decisions, decisions...
More details will be added as work progresses...
