These finer grits are very fast indeed – for the first time, I brought home supplies of two separate grades, and if I can find time to work over the next two weeks I should be able to complete grinding and begin polishing next time I go to the class!
It was an interesting class this last Saturday. Not only did I confirm that the #400 grit had left me with an apparently flawless pit-free surface, but there were a few new faces: a woman (who’s name I did not catch) had brought her son along, together with some of his friends. They’re working on a six inch reflector, similar to mine, and are taking turns at grinding. If they can keep it up, and are able to master the technique quickly enough, then the four of them could easily overtake me and have a working telescope before long.
My mirror has now had about an hour’s work under #500 carborundum grit. The change from #400 is minimal – it feels and sounds the same, although when I place the mirror on top of the tool at the beginning of a new wet, the slurry flows much more easily, and for a few seconds there is practically no friction. If I place the mirror and let go, it immediately slides away towards the edge. At the end of the hour, I noticed a few good-sized pits, and a few little constellations of tiny shallow indentations with softer, rounded edges – perhaps a grouping of micro-bubbles? More work is needed. Still, I’m working at such a fine scale now that getting to the next stage involves removing a really tiny amount of glass which doesn’t take long at all. Once i’m done here (hopefully only another hour’s work), I’ll do a few wets with #800, and then have a beer because I’ll be ready for Part Two: Making a pitch lap, and polishing. I’m told that this is the first great test of patience. Well bring it on, I can take it!
Meanwhile, I’ve been putting some thought into the physical construction of the telescope itself. The assumption has always been that it’ll have a cardboard tube, an a bog-standard dobsonian mount. However, I’ve got an ancient german-style equatorial mount sitting in a box under my bed which might just be up to the task. And since I’ve only ever used equatorials, I don’t find them complicated or tricky to set up at all. Alternatively, I’ll build the standard dob mount, on a wedge. Since I’m relatively close to the equator, though, that means inclining the whole assembly at a shade under 60 degrees from the horizontal – probably quite unstable. Still, these are just ideas. I’ll think about them more seriously as I get closer to having a working telescope.