The Comet and the Telescope

[beatrice_otter]

When I was ten, my Dad and I built a telescope with our own two hands. And I don't mean from a kit. I mean, we BUILT that sucker, starting with grinding the mirror. (Yes, you can indeed grind your own telescope mirror. It takes for-freaking-ever, but you can do it.) (If you want to know how, google "how to build a Dobsonian telescope" and "how to grind a telescope mirror" and that should enlighten you.) Anyway, we spent a summer gathering with other amateur telescope makers in town to spend hours grinding a concave mirror by hand, while John Dobson advised and taught astronomy. Then we sent it off to be "silvered" while Dad built the mount out of plywood, a cardboard tube used for pouring cement pillars, a record (for use to help it rotate side-to-side smoothly), and some strips of laminate counter and teflon tape (to help it rotate up-and-down smoothly). We bought the eyepiece (to look into it) and the sight (to help find stuff) and we spent our summers after that going out mountains in the middle of the Oregon desert to gather with other amateur astronomers and spend our nights watching the skies. Our telescope was respectably on the small side of medium at ten inches; the majority of telescopes at these star parties were 10-16 inches. (Some were smaller, at 8 inches; there were always a few ginormous 40 inch behemoths that required a ladder unless the object you were looking at was right on the horizon--Dobsonian telescopes, you look in the top end, not through the bottom.)

I'm home at my parents' place on vacation, and Dad got out the telescope to look at the comet. Unfortunately, the sight broke, but he was able to find it anyway.

Comments

[alexseanchai]

*impressed*

[beatrice_otter]

It's actually not that hard. Tedious, but not hard.

To grind a mirror, you need two pieces of thick glass, both round, one the size of the mirror you want when you're finished, one a couple inches smaller in diameter. You put the big one on the bottom, then some sandpaper grit (no paper, just the grit) on top of it, then the other piece of glass on top of it. Then you rub the smaller piece of glass all over the bottom piece of glass in a regular pattern, stopping frequently to rotate the bottom piece of glass by a little bit. What you end up with is the middle worn away on the big piece of glass (making it concave) and the edges worn away on the little piece of glass (making it convex). If you've been doing any half-way regular pattern, and remembering to rotate the bottom one frequently enough, you will end up with a regular curve. As the amount of curvature gets to what you want, you use finer and finer grits to get a smoother and smoother surface. Then, when you're satisfied, you pack the big piece of glass VERY carefully and send it off to a place that silvers mirrors, and they will coat it. You then discard the smaller piece of glass because it has served its purpose.

The body of the telescope is simple. You get a tube of the appropriate diameter, cut it to the right length (iirc there is some math to make sure it's the right length for whatever the focal length of your mirror is) and cap one end of it with a mount to hold the mirror in place. In the other end of it, you mount a small mirror at a 45 degree angle right in the center of it, and cut a hole for an eyepiece. Light comes down the barrel of the telescope, hits the big mirror you ground at the butt of it, bounces back up to the small 45 degree mirror at the nose, and bounces from that to the eyepiece you look through.

You build a plywood box around the tube, near the bottom, and put a two circles (plywood or PVC) on either side, and then put some relatively smooth stuff on the circles. Dad and I used scraps of laminate countertop he had from some counters he'd made. To hold that, you build a platform: a square of THICK plywood or mdf, with something slick to rotate it on (we used an old record) and a three-sided box that rotates side-to-side on top of that. (It's three-sided and has no top because the telescope sits on top of it, and you need to be able to point the thing straight up so it has one side for stability and one open side so it won't get in the way.) At the top of the box you have a curve cut on each side that matches the curve of the circles you put on the side of the barrel of the scope. Then you put the barrel of the scope on top of the base, circles resting inside the curves, and voila, you have a telescope.

None of these steps are very hard, if you have the instructions, it's just grinding the mirror that's tedious because it takes so loooooooooooong.

[heartofoshun]

Wow~

[beatrice_otter]

It's a lot easier than you think, if you have the instructions. It's mostly just tedious.

[sebenikela]

I SO wanted to do this when I was a kid, but my parents are not the kind of people who enthusiastically jump into this kind of project so it never happened. Maybe I should add it to the list of "things kid-me wanted that grownup-me can now do" and see if I get to it.

[beatrice_otter]

You totally can, it's not that hard, it just takes TIME. Much of which is spent doing boring repetitive stuff so I recommend either doing it with a group (as we did) so we could chat while we worked, or doing the grinding while watching TV or something.

No idea of the cost, we built it thirty years ago and I was a kid, not paying attention to the price of anything.

[lokifan]

That's so cool!!

[beatrice_otter]

Thank you!