It is official – my mirror is done grinding. Although I had planned to stop at #800 grit, I found myself with time to kill at the last class, and decided to give it a few wipes with #1000. I spent about 45 minutes working with this stuff, so that I’m confident my mirror is now a perfect spherical section, within a tolerance of about 8 microns (since that’s about the average size of individual grains of #1000 carborundum). Which is pretty impressive, considering that this is done by hand, with no instrumentation. It blows my mind to consider that these techniques were developed hundreds of years ago, that there was a time when opticians would test their mirrors with monochromatic light created by burning salt in an oil lamp. The art of mirror making has been unchanged (for amateurs, at least – professionals use machinery) for centuries. Sure, for the longest time the mirrors were ground out of a hard alloy called speculum, instead of glass, but the techniques did not change with that transition.
But back to my mirror: I have not tested it in a long while, but it has a diameter of 155mm, and a focal ratio which I seem to recall being around f=6.5. Fine grinding is complete, and I managed to avoid scratching the glass in the process (no mean feat – my workstation at the last class was liberally dusted with what was either iron filings, or carelessly spilled #80 grit. I didn’t investigate, for fear of it sticking to my hands. The effect of one of those falling on my glass while I work would be pretty devastating). So, now I start polishing. Before I can start, I need to collect or build a few items. First a polishing platform, which is something to grip whichever glass is underneath formly. I’m told you overcome a great deal more friction polishing than grinding, and if the work is not firmly anchored, it will slide all over the table. I was shown an example at the class, and it was suggested that I simply use that one, but it wouldn’t work for me: My tool is quite thin, at maybe half an inch. That makes those supports taller than the tool, so that I wouldn’t be able to move the mirror at all (Unless I was making 1mm strokes…). Instead, I will cut a square of chipboard from one of those incredibly useful offcuts and scraps that all men keep lying around, and make retaining pegs from some rubber washers I have left-over from the last time I tried to fix some plumbing. I’ll stack them up two at a time (for a peg about 8mm tall), and screw them down into the board. I also have about a litre of leftover white enamel paint which should be salvagable, and will use this to seal the wood with about 3 or 4 coats before screwing down the pegs. Finally, I have some rubber mats from my car (I removed them because they slide around from day to day and like to bunch up under the clutch pedal. Irritating), which I might attach to the bottom, or simply place under the board, to ensure that the board itself grips the table.
The next item I need is a plastic bottle – the sort used to dispense vinegar or tomato sauce in restaurants. This will hold my cerium oxide slurry, and make it easy to apply whenever necessary.
And finally, it’s time to start working on a mirror cell. The literature is full of talk about complex multi-point flotation systems, active support, slings.. but my mirror is small and it’s thick. I’m pretty sure it won’t flex, and I live in a country where indoor temperatures seldom differ from outdoor temperatures by more than 20 degrees. I don’t need to think about airflow to stabilise temperature quickly. So my design will be a simple one: Flat round plywood base, with a radius several centimeters larger than the mirror. A circular ring will be attached, with an inner diameter as close to the radius of the glass as possible, forming a recess into which the mirror will fit snugly. Line this recess with felt (or possibly some other material to cushion any irregularities in the plywood), and fit a few retractable retaining clips to prevent the mirror from ever falling out once it’s installed. Simple and easy, if my woodworking skills existed… still, I know enough people who can help, so that shouldn’t be a problem.