And we’re back
Last night I made time in my schedule to get in a bit of grinding. Not a lot to say, really, except that I spent about 20 minutes restoring a very thin bevel to the edge of mirror and tool – maybe 1mm, if you’re feeling generous. Time will tell if it’s generous enough, but I don’t want to sacrifice any more aperture than necessary!
I then put in about an hour’s grinding with the #220 grit. Progress seemed to go slowly, and I took a break in the last 15 minutes to dry the mirror and inspect for pits. This is a bit of a painful exercise for me, since it can be difficult to remember the position and size of pits to be able to see how the surface is progressing. But, I tried out an interesting trick I saw from one of the various ATM mailing lists I follow, which was to mark each pit with a permanent marker. What you do is light up the glass brightly, so that the pits show up as bright specks on the freshly ground surface. You then mark them with a generous thick black mark. Repeat for as many pits as you can find. Then resume grinding. A single wet should be enough to clean off all the marks (assuming you’ve got good contact between the mirror and the tool), but here’s the clever bit: The insides of the pits, being recessed from the surface by whatever fraction of a millimeter, are still coated with ink. Now when you hold the glass up to a light, those pits show up very clearly as tiny black spots. Suddenly it’s very easy to monitor pits – once all the black spots are gone you can be sure you’re eliminated most of the pits from the previous grit.
This test gave me a pleasant surprise: There are very few pits remaining on my mirror! Perhaps five still to go. So I will continue till they are gone, then put in another hour or two with my remaining stash of #220 grit (just to be sure), and then move on to the next grade. This is very exciting, and shows that I’m making real progress! I think it’s time to start planning the rest of the telescope – materials, design, etc. Can’t wait!
I then put in about an hour’s grinding with the #220 grit. Progress seemed to go slowly, and I took a break in the last 15 minutes to dry the mirror and inspect for pits. This is a bit of a painful exercise for me, since it can be difficult to remember the position and size of pits to be able to see how the surface is progressing. But, I tried out an interesting trick I saw from one of the various ATM mailing lists I follow, which was to mark each pit with a permanent marker. What you do is light up the glass brightly, so that the pits show up as bright specks on the freshly ground surface. You then mark them with a generous thick black mark. Repeat for as many pits as you can find. Then resume grinding. A single wet should be enough to clean off all the marks (assuming you’ve got good contact between the mirror and the tool), but here’s the clever bit: The insides of the pits, being recessed from the surface by whatever fraction of a millimeter, are still coated with ink. Now when you hold the glass up to a light, those pits show up very clearly as tiny black spots. Suddenly it’s very easy to monitor pits – once all the black spots are gone you can be sure you’re eliminated most of the pits from the previous grit.
This test gave me a pleasant surprise: There are very few pits remaining on my mirror! Perhaps five still to go. So I will continue till they are gone, then put in another hour or two with my remaining stash of #220 grit (just to be sure), and then move on to the next grade. This is very exciting, and shows that I’m making real progress! I think it’s time to start planning the rest of the telescope – materials, design, etc. Can’t wait!