ATM (Amateur Telescope Making)

Fine Grinding The 22" Mirror
by Jerry Wright

(Scroll down for photos)

After completing generation of the f/3 curve and smoothing to 220 grit with the 6.5" steel tool, I cast plaster tools for fine grinding, polishing, and figuring.

It was too damn cold to do much work in the garage over the winter. I want to keep the cost of the mirror to a minimum, and running the electric space heater full time would have cost too much.
There were a few warm enough to make a drying oven for the plaster tools, and to cast the tools. A full size polishing tool, 1/3 and 1/2 diameter figuring tools, and a 5/6 diameter fine grinding tool.
The tools were dried and sealed with spar varnish.

On April 30 2015 I started fine grinding by hand MOT on the 18.33" tile tool.  This was so That I could see through the back of the mirror how good the contact was on the tiles and that there was no proud tile near the edge of the tool that might snag the edge of the mirror during stroking. Working by hand I could stop the stroke at the least sign of snagging. After 15 or 20 minutes I was satisfied that I could put the mirror on the machine and safely work TOT.

Machine work was done at 8 table rpm and stroke rate of about 1.6 strokes per minute. Stroke length was 5.5" and stroke ellipse centered about 5" from mirror center.

A total of 20 hours of work was done with "30 micron" grit, and all blemishes removed from the surface. It was the best surface I had ever seen after "30 micron" grit.

I ground with 15 micron grit for 4 hours.

As I ground with 15 micron there were a few things that made me think that maybe the 15 micron grit was larger than the 30 micron. I inspected all my micron grits (30, 15, 9, and 5) with a microscope. Some of my 5 micron grit was labled as 30 micron. So after finishing 220 grit with the small steel tool, the 20 hours of work that I thought was with 30 micron was really with 5 micron.  I could have started polishing, but I ground with 15 micron.

A little setback but not all bad.  I learned that I could go to 5 micron straight from 220 and get a good grind in 20 hours on the machine. To me that is easier than cleaning the machine and tile tool after each of a few different size grits. From now on with the machine it will be 5 micron after 220.

After 15 micron I ground with 5 micron for 4 hours.  The surface was again free from blemishes and ready to polish.

Plaster tools of various sizes.

Milling the back of one of the plaster tools.

Finished 18" tool.

18" tool with ceramic tiles expoxied on (seen here before fine grinding).

Hand grinding with MOT (Mirror-On-Top).  The 18" tool below is visible through the wetted back of the mirror.

18" tool after fine grinding.

Chip is no problem--adds character.

Mirror ready for polishing.

Foam protection for the mirror.

Mirror with protection applied.