Back at it again. This will be a good winter project to complete, finally. I am almost complete with the 6" mirror grinding, in the final stages of polishing, and then a dose of figuring and off to get the coating (no, I have no interest in constructing a vacuum chamber to coat this thing) and we'll be ready to complete the rest of the scope.
So last night I made the new pitch lap, with Gugolz 55 instead of the old Burgundy Pitch Lap that was not working quite as well as I wanted, due to the extreme cold temperatures in my basement during the winter. The new lap looks a little rough, in addition to the vast visual change in color from a light brown to jet black, which will be strange at first, but looks don't matter, only performance, so time will tell.
The process is pretty sketchy. There's a lot of "hurry up" time involved, after an hour of slow cooking the brick of pitch to melt it. Then after poured you have to rapidly peel the tape wall off the tool and smash the mirror into it to set the shape, then cut a few channels for drainage, and repress for perfect shape. All that while the pitch is rapidly cooling. As soft as it is (very taffy-like) it shatters extremely easy. I unintentionally chipped a few areas off that I wish wouldn't have happened, but hopefully the rest of the lap will perform well.
- Bill Mitchell 11/3/09
