The starting materials: One six inch mirror blank (made from recycled glass), one six inch tool (same glass, only thinner), two tubs of 80 grade carborundum grit, a spray bottle of soapy water, and lots of newspaper. Not shown: A bucket of water for wetting and cleaning the glass.
About an hour's grinding has been completed. Mirror and Tool have been washed, and some fresh abrasive added to the mirror. Before resuming grinding, the abrasive was then sprayed with soapy water and spread around the surface of the tool
Wet surface of the tool after about an hour's grinding. Note how uneven the surface of the glass still is. This recycled glass is wavy stuff!
Surface of the tool after a wet has been completed, before rinsing. The pale grey sludge is a micture of ground glass and broken down abrasive. This stuff is quite sticky and tends to grab the mirror while grinding, so that the tool slides around the workspace. Not a huge problem though, because there's the grit has pretty much stopped grinding by this point.
At the end of the first day, this was my progress. The mirror, right, has a completely ground surface (depth of 0.70mm so far). The tool still has some untouched areas. Both discs need to be beveled, to reduce the risk of glass chipping from the edges.
At this point, both discs have been beveled twice (once unsuccesfully with regular sandpaper, once with 100grit waterpaper. Result: just under 2mm bevel around both perimeters. This reduced the measurable depth quite a bit.
Still working with #80 grit, the surface of the tool is almost completely touched. Current depth is now 0.63mm (measured with feeler gauge and a straight edge). The apparent flaws on the mirror surface are just patches of moisture, from handling the mirror with damp hands.
Finally! 90 minutes into the second ATM class, the tool (left) is finally ground on 100% of the surface. As it happens, at this point the depth of curvature is 1.00mm - ready to move to #120 grit and start getting those surfaces sperical
Look at those little chips. This is what happens to an unbevelled edge. Before I do anything more, I must grind a 45 degree bevel all around that edge, and do the same for the tool.
The tool also has tiny chips around the edge. Fortunately they're small enough that they'll vanish when I grind in a fresh bevel
The glass provided by the ATM class, hosted by Johannesburg center of Astronomical Society of South Africa (ASSA JHB), is etched with a serial number. The tool is disc #5102
The mirror's serial number
The mirror has now been beveled, with 160 grit waterproof sandpaper. This is about an hours work, by hand.
The bevel on the tool was made with 100 grit waterpaper. After the photo, I gave it an extra few minutes with 160 grit paper, because I have switched to #120 grit for grinding
The surface is a lot smoother (compare with the bevelled edge in this and earlier photos) after about 4 hours of grinding with #120 grit, but there are still quite a lot of pits left over from the previous stage. They're not obvious in this photo, but can be seen by holding the glass over a bright diffuse light.
Nearing the end of the #220 grit stage, hunting out the final pits. Here I've inked them with a permanent marker. The idea is that the ink will be scrubbed away as I grind, leaving only the recessed regions of each pit coloured in. Future inspections will then be very easy, as old pits show up clearly as black dots against the mirror's surface.
Two pits - one marked black from the previous session, and one un-inked, showing bright white (out of focus, bottom right). My thumbprint provides a scale to judge the size of the pits. Note that the surface is clean. The fact that the glass appears transparent under the thumbprint shows how small the pits left by #320 grit are - the thin film of oil from human skin is enough to fill them in!
The surface after a few hours grinding with #320. Not the very large pit in the centre (coloured black from a liberal application of a permanent marker. One wet grinds away the ink from the surface leaving only the pit black). This must be either a #80 or a #120 pit.
One smaller pit top left, near the thumbprint.
Another pair of pits - one marked black, and one unmarked near the edge. You can see what little remains of the bevel on the mirror. It is actually too narrow, but I'm hesitant to grind in a fresh one now that I'm at this fine grit level, for fear of scratching the surface.
View through 40mm eyepiece used as Loupe, of mirror's edge. Visible roughness on what remains of the bezelled edge, but surface totally pit-free, ready to move on from #400 grit.
View through reversed 40mm eyepice, of mirror surface at centre. Totally free of pits, I have finished grinding with #400 grit, and am ready to move on to the next grade
My mirror being tested for pits. The light beneath the mirror illuminates any surface pits brightly, while the makeshift loupe magnifies your view enough to see the smallest pits. In this photo, the pits were almost too small to see, as I had almost finished with #800 grit.
A six inch polishing tool, wet with cerium oxide slurry. The tool rests on a piece of bubblewrap (which was trimmed after the photo was taken) to ensure an even pressure against the board, preventing any flexure.
The tool is loosely gripped by three rubber pegs (made by stacking rubber washers) to prevent it sliding around under the high-friction work of polishing
The mirror on top of the pitch lap, ready to polish. The surface is so finely ground that a little bit of moisture makes it appear to be already polished.