Cyde Weys Musings

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.

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On Friday I continued working on my primary mirror at the amateur telescope making workshop in Washington, D.C. I finished grinding the 8″ blank with 60 grit, then 80 grit, then finally 120 grit silicon carbide. Guy, who leads the workshop, must have detected my eagerness to complete my first scope, because he asked me if I wanted to take it home and work on it over the week. Of course I said yes.

So he packaged up some 220 grit, 320 grit, and 400 grit aluminum oxide for me and I brought home the glass blanks and worked on them yesterday out in the backyard. I finished up with the 220 grit but I had to stop at that point. My arms and back were absolutely killing me (and still are). After four hours of rigorous grinding on Friday and another hour on Saturday, I’m totally shot. My entire upper body is sore beyond belief. Making a telescope yourself may save money, but it certainly requires a lot of effort. It’s good exercise too.

The workshop wasn’t as well attended as it was on Friday. Oddly enough, I didn’t see a single person who had been there the previous week (besides Guy, of course). It must be a sporadic activity, with most people attending as often as they have Friday nights free. A telescope project can easily drag out over the course of many months in this manner. But I hope to be finished much sooner. And I remembered to bring a camera this time.

The first photograph (used as the lead-in to this post) shows the two 8″ pieces of glass I’m working with. The one on top is what will become the mirror, and thus it has a concave (curved inwards) surface. The final step with the glass, which I’m a long ways away from, is aluminizing its surface, thus turning it into a mirror. The piece of glass on bottom is the “tool”. It has a convex (curved outwards) surface that matches up with the concave surface of the mirror. The two are ground together with water and 80 grit silicon carbide in between in a specific back-and-forth motion that creates the desired shapes (slowly). As you can see, it’s a messy task, hence the newspapers.

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On Friday night I went out and had a lot of fun. It wasn’t the typical kind of fun that most people associate with going somewhere on Friday night, but I daresay I enjoyed it even more. I started building my own telescope (a program put on by the National Capital Astronomers). Before I go any further, let me just get this out of my system: How awesome is that?! There are all sorts of awesome things you can do; you simply have to go and look for them. For me, that awesome thing was building a telescope, something I’ve wanted to do for a couple years now, and I’ve finally gotten started.

The telescope making workshop is held every Friday night from 6:30 to 9:30pm in the basement of Chevy Chase Community Center in Washington, D.C. Ten people were there in total, which is a surprisingly large number considering how esoteric the activity is. For comparison’s sake, that was about the same as the number of people in the juggling class I walked by that was being held in the activity room on the first floor. People were involved in all stages of the telescope-making process, including one guy who had fully assembled his 8″ scope and was tweaking the alignment, a guy making a monster 18″ scope checking how close his 47-pound primary mirror was to a parabola, and a guy who was just starting to grind down his mirror. That’s where I started.

After chatting up some of the other folks at the workshop and asking their advice, I decided on making an 8″ f/8 reflecting telescope with a paraboloid primary mirror (the simplest design). The materials cost was only $100, which is a bargain compared to how much a commercial 8″ scope would cost (go search, I dare you). Also, the quality is surprisingly better with a homemade scope. They have tools at the workshop to get the shape of the mirror to an accuracy of one-quarter of the wavelength of green light from an ideal paraboloid. Mass-produced commercial scopes don’t even come close.

The first step in making a telescope using this process is grinding down a 8″ glass blank. They use Pyrex at the workshop instead of plate glass because it has better thermal characteristics and is much harder (which also, unfortunately, means it takes longer to grind). The guy running the workshop, who is fittingly enough named Guy Brandenburg, helped me calculate the amount of curvature I would need to produce an 8″ mirror with a focal length of 64″. That turned out to be 0.029″ of indentation in the center of the lens as measured by the spherometer (which has a radius of about 2.9″, so it doesn’t go all the way out to the edge of the blank).

All I did for the rest of the night was grind, grind, grind. I had two 8″ blanks, one which would become the mirror and the other which was the tool. To create the initial spherical curvature on the mirror, one places the tool on the table, the intended mirror above it, and grinds it in a circular motion for hours. I was using water and 60 grit silicon carbide placed between the two glasses, though as it gets closer to the correct shape I’ll have to start using finer abrasives, and eventually, a polishing solvent. Once the blank reaches a spherical shape, the next step is to carefully turn it into a paraboloid. But I didn’t get that far in a single night. My blank started with 0.015″ of indentation in the center and I got it to 0.026″ — most of the way towards completing the initial step.

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