Playing Galileo on a Friday night

Schmidt-Cassegrain reflecting telescopeOn 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.

The grinding wasn’t actually as bad as it sounds. I daresay it was more fun than grinding in an MMORPG. It’s certainly much better exercise. By the end of an hour I had worked up a sweat. It also helped that I wasn’t the only one doing it; don’t activities always just seem to go faster in groups? On the other side of the table a youngish couple were doing the same thing I was. I talked with them a good bit. They turned out to be nice and interesting people, so next week I’m not just looking forward to making more progress on my scope, but also, meeting them again.

You may be wondering how grinding two initially flat pieces of glass against each other in a circular motion (and being sure to turn both regularly) produces a perfectly spherical surface. I’m not quite sure of the mechanics myself, but I do know that it works. The glass on top becomes more concave and the glass on bottom becomes more convex. Once it reaches a spherical shape, the stroke changes from a circular motion to a forward-and-back motion (with regular rotation of both blanks) to reach that final paraboloid shape.

You may also be wondering why I’m grinding glass if I’m making a reflecting telescope. That’s simply because it’s the best way to make the proper shape. The glass won’t be used as a lens, of course. The final step after verifying that it is a proper paraboloid is to aluminize it, which coats it with highly reflective aluminum to turn it into a mirror. After that, it’s simply a matter of mounting it in a long black tube and aligning it properly with a deflecting mirror near the front of the scope and eyepiece. Building a mount for the scope comes last.

Just going to the community center filled me with a little bit of sadness. The place is run down and isn’t used nearly to its potential. The workshop we are making telescopes in used to be a functional woodshop, but hasn’t had an instructor in two decades, and is now mainly used for making telescopes once a week and a drumming class. I lament the death and decay of cultural and community centers across the United States. They aren’t utilized to near the same degree they used to, and newly built communities aren’t even bothering to construct community centers. You’ve seen them; they’re just row after row of cul-de-saced houses and strip malls.

Something bad is happening to society. People are driving to work in the morning, driving home in the evening, then holing themselves up in their houses with their electronic gadgets. People don’t regularly see other people in society the way they used to. Community centers are amazing places — where else are you going to go to learn juggling, how to make a telescope, try woodworking, learn to dance, or any of the other hundreds of activities that take place in them, all for an extremely low cost? Community centers are dying out and the services they provide aren’t being replaced. It’s a very sad state of affairs. My advice is to locate your nearest community center, check out the schedule of events, and go to one thing a week that interests you. You’ll meet all sorts of interesting people and you’ll have fun learning to do something new. I’m definitely having fun making this telescope.

2 Responses to “Playing Galileo on a Friday night”

  1. Darmok Says:

    There’s nothing like making things yourself…

    I, too, see the disappearance of community in general. In part, for that reason, I want to buy a bike, so at least I can interact with people as I ride by…

  2. Ivan Krastev Says:

    Dear Ben
    I am the author from telescope design software MODAS and the editor of ATM Letters Journal. Today by searching in the web I found your stuff about telescope making. I think your article will be interesting for the ATMLJ reader. If wish you can submit the article to the journal column “ATM Praxis”. A copy of the complete issue with your article will be send to you and your friends.

    best regards