brainwagon

For the second week in a row, I carted my body down to the Chabot Science Center and attended the Chabot Telescope Makers workshop. My intention was two-fold: I was going to extract the mirror from a little 6″ telescope that I had found at a garage sale and purchased for a mere $15, and then I was going to make a pitch lap for the 6″ f/6.4 mirror that I had found in my garage, hoping to get it refigured to a better sphere.

I used to make pitch laps all the time for the attendees of the workshop: over the years, I’m sure I’ve done hundreds. But I was frankly a little rusty with this one. I got some Gulgolz 64 pitch (Rich, I still owe you for that!) and melted it fairly slowly over a little electric burner outside on our loading dock. The temperature outside was a bit cool, and I was alternating between doing this and trying to help a couple of new students get started on their mirror grinding, so I was a little bit less attentive and more distracted than a guy who hasn’t made a lap in five years should be. The pitch melted nicely, looked great, and I poured it onto a piece of 0.5″ thick flat Pyrex that I had lying around. I let it cool fairly well until it was pretty stiff, and then tried to press it out. To do this right requires a bit of timing: and I think I let the lap get a bit too stiff, so I didn’t get good contact over the entire mirror. I slid the mirror off, channeled it, and then pressed it again, but as you see from the picture above, there is still a small area which is not in good contact. I ran out of time, so I just wrapped up the pair, and brought it home. My idea was that I would take some extra time to clean up the channels, and then do a little bit of warm pressing and get the lap in shape for next week.

Of course, sitting overnight was not the best idea: this morning I’m having some difficulty separating the lap. It’s currently soaking in a water bath, and I’ll give it a try in a bit: the friction is really large and I haven’t been able to budge it yet. I’m not panicking though, I’ve been to this rodeo before, and always managed to separate them, sometimes with a little time in the freezer.

Oh, and the 6″ garage sale telescope? It’s mirror is made from that greenish soda lime plate glass. The coating is a bit rough (it had a lot of bugs on it from spending time in my garage) and looking at the figure, it’s pretty much a sphere, with just a hint of correction and a turned up edge (shocking, turned up edges are less common than turned down edges). I think there is also a hint of astigmatism: the lines of the Ronchi test shifted by about 5 degrees inside and outside of focus. It’s radius of curvature is about 59″, making it just slightly short of f/5. Definitely worth redoing. That will be project #2 (probably can reuse the same lap, once I am done with the first mirror).

My intention was to shoot some video of the lap making process, but the hectic nature of the workshop made that pretty difficult last night: I was flitting around trying to help, as well as get some of my own projects done. I’ll have to figure out how to document some of this a bit better, as some of you have expressed some interest in telescope making. I do realize thus far that you are all probably at sea with respect to what I’ve been writing about it, since I haven’t taken the time to introduce the vocabulary and process. Perhaps I’ll try to do some introductory posts going forward, pulling on material from my old, defunct telescopemaking.org website. Stay tuned.

Addendum: Sure enough, a short 30 minute soak in water, and enough seeped in to the edge that small push separated the lap and mirror. I’ll clean up the channels a bit more, and get it pressed out in the next week or so. Then, onto some judicious figuring. It’s been a while since I’ve done any of that, so I’m planning for a little bit of back and forth until I get my figuring mojo back, but it shouldn’t be that difficult.