Slow and steady wins the telescope-making race

A Ronchi test on my mirror as of 2007-11-16
The amateur telescope making (ATM) workshop wasn’t open yesterday on account of Thanksgiving, but I still haven’t written up a progress report yet on last week’s workshop, so I’ll go ahead and do that now. But first, allow me to talk about the others in the workshop, because there are some fascinating people there without whose help I would not be able to go on.

The workshop has been getting really busy lately. Guy, the guy who runs the workshop, says this always happens around autumn. I believe it. Last week I counted a record sixteen people at the workshop (it had been as low as four a month prior). Many people there are just starting on their projects, while others are returning from long absences to finish theirs up. A father is making a 8″ scope with his young son. A young man in the Armed Forces is making an 8″ scope, and last week his high school aged (?) friend joined him, making his own 6″ scope. There are also several older people making scopes, one of whom is quickly catching up with me.

Then there are the veterans, the people who’ve done it all before, multiple times, and just show up to hang out with friends or make steady progress on their projects in the company of others. One man has at least ten different mirrors and lenses under various stages of construction. He’s also built a mirror-matic at home that he uses for rapidly hogging out and polishing mirrors (it can be done faster than a human with a lot less effort).

Another man, an electrical engineer, is currently working on the automatic grinder in the workshop, which was built and then donated by a previous person at the workshop who gave up on their mirror. The automatic grinder needs a lot of work, but it is salvageable. He also likes singing and playing piano, and he’ll occasionally entertain the rest of us by playing on the piano in the workshop (it doubles as a music instruction room; the piano sits right next to the vaccum chamber).

And then there’s Bill, who seems to find a sense of zen in telescope-making. He polishes really slowly, trying to create the perfect mirror with no imperfections at all. But he doesn’t even keep the end products of his work. He donates them to Guy, who either sells them to raise funds for the workshop or uses them in telescopes for a weekend program he teaches at for inner-city DC youth. Guy recently built three brightly-colored telescopes that he uses to get the kids interested in science. As far as I know, the mirrors were all donated by Bill. Bill has been trading blanks with Guy to build more mirrors out of, and he’s also taken on a few mirror projects abandoned by people who stopped showing up at the workshop long ago, leaving their projects unfinished.

So that’s the story of who shows up to the workshop. It’s a really great group of people, with a nice mixture of ages. Everyone is there because they want to be, not because they have to be. It’s thus a much different atmosphere than, say, school or work.

And as for my 8″ f/6.15 Dobsonian, I’m still making very slow progress. Despite using really wide and long strokes (which aren’t making the edges too good), I’m stuck at around half-way between spherical and paraboloidal, as confirmed by repeated Foucault tests. We think the problem is that the glass is not making good contact with the center of the pitch-lap, which we’ve repeatedly been trying to reshape using a heat lamp and hot pressing. It’s getting better, but it’s not quite there yet. The four pitch squares in the center of the lap are still making minimal contact.

Hopefully once the lap is fully conformant, it shouldn’t take too long at all to get my mirror to the perfect paraboloid. In the picture above of a Ronchi test, you can see that the lines are curving outwards from the center. That means that my mirror is past spherical towards paraboloidal. They just need to bend more to be the correct paraboloid for my mirror. Also note the turned-down edge, identified by the rapid curving away of the lines at the edge. If the edge was perfect, the lines would continue in the same shape.

Other than the turned-down edge and the wrong curvature, though, things are going well. My mirror’s polish is very smooth and even. The turned-down edge can be beveled off, creating only a minuscule loss in light-gathering ability. So getting the correct curvature is only a matter of time. It could be a lot worse. That’s why I’m not giving up — the end is in sight, it’s just taking a bit longer to reach it than I would have liked. Next week, I’m going to start constructing the telescope body itself in parallel with the final figuring on my mirror in the workshop. Hopefully, I’ll have the telescope housing done by the time the mirror is aluminized, so I’ll be able to drop it in and go. I won’t be in time to observe Comet 17P/Holmes, but at least I’ll make the Mars opposition.

2 Responses to “Slow and steady wins the telescope-making race”

  1. Guy Says:

    Quite a good blog, Ben!
    I think that next week we might try melting a bit of pitch and pouring it into the middle section of your lap.
    BTW – you really need to start making the rest of your telescope, if you are going to see Comet Holmes!
    Guy

  2. Cyde Weys Says:

    Thanks Guy. And yeah, I was thinking about adding pitch to the middle – I just didn’t realize it as actually done. I don’t know if I’ll be able to attend next Friday’s workshop though; I’m having two wisdom teeth out that afternoon. If I do show up, at least that will be a good sign of my dedication ..