III. Optics

3.1 Mirror, Grinding and Polishing

The the first part of the telescope I began working on was the mirror. The mounting would wait until a rough design was completed, enough money was saved, and parts were collected. We decided grinding the glass by hand would be a formidable task.  The work would be easer if we had a grinding machine. Bill sketched an initial design for a single arm more conventional gringing machine, Figure 3.1.1. Many of the features of the sketch were incorporated into the

        

            Figure 3.1.1. An early concept sketch for a mirror grinding machine.

two armed version. I always find it interesting to view the sketches and compare them to what was actually built. The actual grinding machine was built with two pittman arms.

The two pittman arms of the grinding machine were fabricated out of some scrap that we got from a door making machine. The over arm was fabricated from a piece of steel tubing with a ten speed bicycle gear cluster for providing a variable speed on the grinding tool. A potato peeler tub was given to me by Jim and was used for the mirror turntable. The pittman arms were supported with a steel tube, the base of which contained a right angle gear box made with the spider gears of a Ford Falcon. A shaft was used to drive the pitman armes through the gearbox. A 1/4" horse motor was used to drive everything through a vari-drive that used a stick shift lever from a truck to change speeds, Figure 3.1.2.
 
 

Figure 3.1.2. The grinding machine built to grind and polish the 22" mirror. The two pittman arms that drove the over arm can be seen at each end of the grinding machine table. The kid's pool was used to confine the water and grinding compound. Three of the sub diameter polishing tools can be seen in the foreground. The stick shift for changing grinding speeds can also be seen.

 The mirror came to me as a 20" clear aperture f6 sphere. The grinding machine was originally put in Bill's basement, where I did much of the rough grinding. Rough grinding was initially done with 60/90 grit to remove the roughly 0.070" for a final sagitta of 0.320". It was inconvenient to go to Bill's house to work on the mirror while studying. We moved the machine to my apartment dining room to finish rough grinding, fine grinding, polishing, and figuring. The grinding machine literally took up my entire dining room. The move increased the amount of time that I had to work on the mirror. I could now study and work on the mirror at the same time.

I put a bucket with a pump under the potato peeler for the water from working the mirror to flow into. When the bucket was full the pump would allow the water to be emptied into the kitchen sink. I attached a plastic tube with an air nozzle on the end to provide a source of water for working the mirror. The mirror was completed in about 1984. It took approximately 250 hours to grind and polish the mirror. The mirror was figured to a smooth parabola of about 1/4 wave.
 

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