The right tools are essential for any project, and it’s understandable to think that that the tools needed to make a precise optical instrument like a telescope must be precise instruments themselves. Surely to shape a micron accurate curve into a piece of glass can only be achieved in some expensive laboratory beyond the reach of mere mortals? Not so!
The first telescopes were made in the early 17th century at a time when work at night was done by candlelight and horsepower was just a horse. Scientists and astronomers often built their own equipment, and a low-tech solution was the only solution. Nowadays most of the items needed to grind a mirror are available from DIY and craft stores.
The curve of my mirror will be ground into the surface of a Pyrex disc using a range of coarse to fine grits – think sandpaper without the paper. The tool that I’ll use to grind the grit against the Pyrex must be the same size and shape as the disc, have a surface of similar hardness, and be waterproof. Given these requirements the simplest answer would be to buy a second Pyrex disc and use that as a grinding tool. In fact some people still use this method for smaller telescopes, but larger sized discs of glass or Pyrex are expensive and so it is preferable to make a tool out of cheaper materials. I’ll be making mine with plaster, bathroom tiles, epoxy, and varnish.
The easiest way to get a tool the same shape as the disc is to use the disc as part of the mould. I covered the front of my Pyrex with cling film, and then wrapped a slip of cardboard twice the thickness of the Pyrex around the disc to act as a dam. I then filled the mould with plaster, and after an hour drying removed it to cure. The back surface came out pretty lumpy, but the front surface that faced the Pyrex is the one that really counts and that came out nice and smooth. Such a thick chunk of plaster took a week to fully dry and cure, and then a little bit of sanding leveled out the back surface and took off sharp edges and mould marks.
Plaster is too soft to use for grinding, and so the next step is to put a harder surface on the front of the tool. I’ve used unglazed porcelain mosaic tiles. One of the keys to mirror grinding is randomness, and so it is important that the tiles are not centred on the grinding tool. To test the distribution before gluing, I drew an off-centre grid on the front of the plaster and arranged the tiles to make sure they fit well on the surface.
Happy with that, I put on a first coat of epoxy to ensure the front of the tool would be tough and waterproof. After an hour drying I applied a second coat of epoxy and – quickly! – laid the tiles into the surface.
After another hour to dry and with the tiles safely in place the rest of the tool needed to be waterproofed. While grinding the tool will be regularly rinsed with water, and if even a little water gets into the plaster it could start to disintegrate! The two coats of epoxy provide good protection for the front of the tool, but to waterproof the back and sides I applied four coats of outdoor wood varnish.
The result looks like a cross between an unappetizing cake and a poorly constructed computer chip. I may not be winning bake-off, but I think I’ve got a sturdy washable tool to start grinding my mirror!