Third week of amateur telescope making

Telescope 39
I’ve made a lot of progress on my telescope since the second week. Just as a recap, I’m aiming to make an 8″ f/6 Dobsonian telescope, by hand. I’m still working on the 8″ diameter disk of Pyrex glass that is going to be my primary mirror. The last step is making it reflective by coating it with a layer of aluminum. I was at the workshop on Friday night for 3.5 hours. I finally saw some repeat people who had been there previously (besides the Guy running the place).

My night was fairly boring. I worked with 400, then 500, then 700 grit abrasives (the number is how many particles it takes to make one inch across). The particles are small enough now that it’s really important to make sure that they are always kept wet, so the dust they give off when ground up isn’t inhaled. There’s a famous astronomer who died of a really bad lung disease after making too many scopes and inhaling too much silica dust. His name escapes me at the moment.

The exciting part of the night came when a man who drove down from New York had his mirror aluminized. The amateur telescope workshop up there doesn’t have a vacuum chamber. He disappeared to the kitchen of the Community Center for an hour, thoroughly washing and re-washing his glass, making sure to get rid of every last bit of contamination. Then they started up the vacuum chamber.

The vacuum chamber is an old Navy relic that was originally used for God knows what. When it was too old for their purposes they donated it to American University, and then when it was then too old for their purposes, they donated it to the National Capital Astronomers, where it ended up in the amateur telescope making workshop. It’s not in bad shape though. It’s typical of military hardware: rugged as all hell, made from steel instead of wimpy plastic, and it does its job for its intended lifespan and then decades more.

It’s fascinating to watch the vacuum chamber in action. First, the mirror is loaded, upside-down, in a harness at the top of the vacuum chamber. Then an aluminum slug is inserted into a tungsten coiled filament at the bottom of the vacuum chamber and the dome of glass is lowered. It takes awhile for the vacuum pump to suck all of the atmosphere out of the chamber. Meanwhile, a diffusion pump is running through cold water coming from the sink, used as a coolant. Finally, the requisite level of vacuum is released, and a current is applied.

This is when weird things start happening. The last vestiges of air in the chamber glow a haunting blue that shines out through the cage. It’s the same cause as the Aurora Borealis, just on a smaller scale. We turned off the room lights to get a better view of the proceedings. Then, the current is turned on full for just a few seconds. The current travels through the tungsten filament, vaporizing the aluminum slug and creating free-traveling aluminum atoms in the vacuum, which deposit themselves on everything inside the vacuum chamber — including, of course, the concave surface of the glass. The process is very very bright, emitting an uncomfortably bright orange/yellowish glare from the bottom of the vacuum chamber. And then, just as soon as it began, it ends.

That’s how it’s supposed to work, anyway. But the filament broke under the strain in less than a second, which had Guy cursing. Luckily, it ended up being alright. A sufficient layer of aluminum had been deposited on the glass, turning it into an excellent mirror. I noticed the woman I met at the first week of amateur telescope making gawking at it. I will admit, I was too. It’s exciting seeing the end product that is only a few more weeks away. It’s also excellent encouragement. I resumed grinding away at my mirror with renewed vigor after that. Even if you’re not making a telescope, who wouldn’t want an 8″ perfectly parabolic mirror? Oh, the paper you could catch on fire with pure sunlight using just that!

After the workshop I took home some 17, 11, and 5 micron abrasive slurry. Manufacturers stop using grit measurements for abrasives this small. It becomes more convenient to store them suspended in water than as a powder. I finished up with those three sizes earlier tonight while watching part four of the excellent World War II documentary series The War by Ken Burns on PBS.

So I’ve now completed grinding with the smallest size abrasive available at the workshop. My next step is going to be polishing. My glass is still spherical, which is the shape that the back-and-forth stroke I’ve been using creates. At some point I’ll have to switch to another stroke, and the use of repeated Foucault testing, to get it to be a paraboloid. I’m not quite sure on the details of that stage yet.

And now, it’s picture time! The lead-in picture shows a neat fractal-esque pattern created when I poured some fresh 5 micron slurry on my glass blank. There’s not much scientific value to this shot, but it sure looks neat.

And here are four pictures I took of the vacuum chamber. I know, I committed the cardinal photographer’s sin and cut off the top of Guy’s head. Whoops!

Telescope 28

Telescope 25

Telescope 30

Telescope 24

2 Responses to “Third week of amateur telescope making”

  1. Darmok Says:

    Even if you’re not making a telescope, who wouldn’t want an 8″ perfectly parabolic mirror?

    I had to laugh at that…even while simultaneously being jealous of those with such a mirror!

    By the way, you accidentally, placed a second <i> after “The War” instead of closing the first tag.

  2. stipe Says:

    You are very lucky to have the vacuum chamber for ATM purposes. Here in Croatia we do not have any chamber for aluminising the mirrors, that is a real big problem here. and there are a lots of amateur astronomers who have a dozen of mirrors ground and waiting to be coated but seems to be never done.