If you have done any woodworking, you know that you usually shape the wood with tools like saws and axes, and once you get close you shift to scrapers, and then proceed with finer and finer grades of sand paper until the surface is smooth and beautiful.
Making a telescope mirror proceeds in roughly the same way: one begins with a very coarse grit and uses it to remove enough glass from the mirror to give it the correct concave shape, and then one shifts to finer and finer grits to remove the pits left by the coarser grits. When you get to the finer grits, you shift to a polishing compound and finish the mirror to a fine polish.
The first thing you need is a mirror blank, which is just the name for a thick round slab of glass. If you are just a beginner, I'd recommend just buying a good quality Pyrex blank from a telescope parts supplier. One can use other types of glass like old portholes that are scavanged from old navy ships, but figuring plate glass can be tricky because it expands and contracts a great deal with changes in temperature. Nevertheless, some excellent mirrors have been made from such scavanged glass, so if a nice 16 inch porthole falls into your lap, don't be too timid.
You don't need a lot of equipment to make a mirror. In fact, it requires so few tools that we use the term "the tool" to refer to the most important one. A grinding tool can be another glass blank just like the mirror blank. This is usually considered wasteful, because blanks are rather expensive, and when you are through grinding, the tool will have a convex surface that isn't useful in making telescopes.
It is more common these days to make a tool instead of using a glass one. Most of these tools are made out of plaster or cement, and covered with a layer of tile. These tiles are inexpensive (they can be made for $10 or less) and grind very efficiently. I can't think of a single reason to recommend using glass tools unless you just happen to have one.
The best instructions that I have found on making tile tools are Berthold Hamburger's site and the AstroPages website. Each person does these a little differently, so don't be too concerned about the differences between the methods that different people use, just pick one and stick with it.
If you are lucky, you have plenty of space in a nice temperature controlled workshop where you can leave a 55 gallon drum weighted down with 200 lbs of sand in the middle of the floor. But if you aren't that lucky, you can still grind a telescope mirror by making a simple grinding board.
The grinding board is really nothing more than a piece of wood with cleats screwed into it to hold the mirror or tool in place so it doesn't skid around. The easiest way to make one is to go to a place which manufactures kitchen countertops and ask for a sink cutout. They will usually charge you some minimal fee (I got my last one in exchange for Starbucks) and hand you a nice piece of particle board which is covered with some kind of formica or other water resistant countertop material. This will make cleaning up after each grit much easier.
All materials on this website are Copyright 2001, Mark T. VandeWettering. Permission is granted to reproduce and distribute these files for non-profit, personal use.
Mark T. VandeWettering <markv@telescopemaking.org>