The ATM class was much busier this time. At least three first-timers, although only one of them seemed interested in getting started. A fair bit older than me, but you wouldn’t think so from the way he attacked that glass – by the end of the day he had ground down to a depth of 1.6mm. Since he’s aiming for the exact same mirror as me (6″, f/8), he now has to perform corrective surgery! Based on the theory I’ve read so far, I expect he’ll be working Tool on Top for quite a while now.
As for my own mirror, the little smooth hollow on the tool gradually shrank away till after about 90 minutes of grinding with #80, it was gone. At last! The two surfaces are now mating completely. Judging from the size of the bubble visible through the mirror while I’m grinding, though, I’ve still got a fair bit of work to do before I’ve achieved a spherical surface.
Two more achievements: First, my carefully sanded bevels are completely gone. That mirror now has a razors edge, glittering with countless tiny chips. Before I do any more work, I need to create a new bevel. On the plus side, that means that I could get an accurate sagitta measurement for the first time in a while. Shortly before packing up for the day, I cleaned and dried the mirror and broke out the spherometer. I have achieved 1.00mm depth, meaning that I am only 0.17mm from target. Johan judged that I should switch to #120 grit immediately, and supplied me a nice stock for work at home. Now it seems likely to me that I will probably overshoot my target depth as I work towards getting my mirror spherical, but that’s okay. I’ve been measuring the various doors in my car and realise that a 1.2m telescope will be a little unwieldy to pack. The boot is a hair over 1 meter wide, so perhaps I should aim for that as a focal length. That gives a conveniently round number as the new focal ratio (f/7), and a new target depth of 1.25mm. Easy!
In summary: 2 hours grinding with #80 has left me ready for the next stage, and a mirror in desperate need of bevelling. I’m feeling quite confident about this project!