Hogging out the Curve with a Pipe Fitting

I started work on the mirror early in the summer of 2001. I had moved to Maine from Newfoundland in January, and (because of tight economic conditions in the computer industry) most of my possessions are still in storage, a few in Newfoundland and most in California (but some happy news - I'm expecting the stuff from Newfoundland in a few days). I had little furniture and no tools and not a lot of money to buy stuff with, but I wanted to get working.

So for a workbench I used a low metal shelf that is bolted to the wall of a room off to the side of my garage. I put some paper towel on it so the mirror wouldn't slip and set to work.

The first thing one needs to do is bevel the mirror, to reduce chipping at the edge. I started by rubbing the edge with coarse sandpaper, followed by filing with a wet file. I don't know why, but I hate beveling, so I never put that much of a bevel on my mirrors. I think a grinding stone would work better than the file. Sandpaper has the advantage of making a curved bevel.

One aims to remove as much material as possible in the initial stage of grinding, to get the mirror from its initially flat shape to a rough curve of the desired radius. Initially I decided I would make the primary f/3.5, so the focal length would be 28 inches, giving a radius (twice the focal length) of 56 inches. This is pretty short as telescope mirrors go.

For a "hogging" tool I used a steel pipe fitting, a reducer with a 2 inch inside diameter on one end and 1.5 inch ID on the other. The lip of the large end is about 5/16" wide. The small end was a comfortable size to hold in my hand. I had planned to use a pipe cap but couldn't find one at the hardware store of the required size. I had also planned to use a 3 inch fitting but the people on the ATM list advised me I didn't need such a large tool for an 8 inch mirror. Someone else had recently rough ground a 13" with a 2" pipe fitting.

I used #80 carborundum (the coarsest grade I had, and the coarsest commonly used for telescope making) and water, with the mirror face up. I ground with strokes straight across the center of the mirror, stopping a half inch before hitting the edge, not wanting to cause a chip.

I understand this kind of tool is called a "ring lap" in more refined circles.

I kept a bucket of water next to me to wash the mirror off periodically.

The grinding went suprisingly fast. The abrasize was also used up at an alarming rate, in part because it broke down so fast and in part because a lot got thrown off the edge of the mirror. The primary is plate glass, and I've mostly ground pyrex before, and I wasn't used to how fast it could be hogged out.

After I expressed some concern about running out of grit on the mailing list, Dan Cassaro very kindly sent me another bottle. I think there would have been plenty if I hadn't been grinding so short a curve. I used the same bucket of water all the way through rough grinding (including finishing it up with a tile tool) and was able to recover quite a bit of abrasive from the bucket, which I'll use when I "trepan", or perforate the mirror.

I've misplaced the notebook I was keeping at the beginning of the summer (I'll find it, it's around somewhere), but I remember that I got it to a little longer than the desired radius in just a couple of hours. The curve was not particularly spherical - it was flat on the edge, then started to bend gradually downward, steeper and steeper, then even out somewhat spherical across the middle. This was to be corrected when I made a full sized tile and dental stone tool later.

I did a bunch of work in about a week, but then I stopped for a while when my consulting work got hectic. I wasn't so sure I'd be able to keep working for my current client for long, so I put a lot of time into hunting for new clients. This was very frustrating - there were still many layoffs happening, and a lot of people were out of work. Happily, in the fall my client agreed to keep me on long-term, so after spending some extra time on his work I was able to go back to mirror grinding.

The next step was to make a tile tool, but unfortunately disaster ensued.