First of all, apologies for the late update. On a whim, I decided to replace Windows 7 with FreeBSD Unix on my PC, and have spent the last few days getting all my software installed, and the environment tweaked exactly the way I like it. It’s looking good. I have to say, I’m impressed – the KDE window manager has moved a very long way since I last used FreeBSD as a desktop operating system 7 years ago.
Saturday I made it back to class (after being absent for two weeks). A few new mirrors had been started since I’d last appeared, and it was interesting to compare them with my own. I’d forgotten how noisy the #80 grit abrasive is – the #120 is much quieter. One of the old hands brought in a commercially produced mirror and had it set up on the Foucault tester and was inviting us all to come have a look at what can be achieved by modern technology. Now, as a disclaimer, I have next to no experience evaluating mirror surfaces with the Foucault test. I know that a spherical mirror should appear uniformly flat, and a paraboloid should have the characteristic “Donut” shape, but there are subtleties to the degree of shading that depend on the mirrors diameter and focal length and interpreting these is a matter of experience, and probably some black magic as well!. None of this mattered, though. This mirror’s surface looked like the surface of a pond about two seconds after somebody dropped a stone in the middle. I’ve heard people describe mirrors as having zones before, without quite understanding what was being said. These “ripples” represented a lot of severe zones – I imagine this could be the worst mirror ever tested by this class! Suddenly the old story of “Your first handmade mirror will outperform most commercial products” seems very believable indeed!
Aside from that educational interlude, the afternoon progressed uneventfully. I ground for about two hours, still on the #120 carborundum grit, checking occasionally for pits by holding the mirror over a frosted lightbulb and examining the surface with a magnifying glass. It has been a little discouraging to constantly find large pits, but Johan pointed out that you don’t expect great quality and consistency with #120 grit, so that many of the pits I see may in fact be brand new. The trick, apparently, is to inspect often and get to know the surface intimately, so that you can recognise new pits and monitor the old ones. Once the pits are always new, the surface is ready.
And the milestone referred to in the title of this posting? My focal length is now about 126 cm – my mirror is almost exactly at f/8. If I was following the textbook pattern for a first time amateur telescope, I would be ready to progress to a finer grit. Unfortunately, such a telescope would not fit into my car, so I have to make it shorter. I am aiming for a focal ratio of f/6.5. It will be more difficult to figure accurately, but it will be worth it to have something that I can actually use!