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Homemade Dobsonian Refractor

Sarah Williams' poem " The Old Astronomer to His Pupil" ends with " I have loved the stars too fondly to be fearful of the night," and kindling a love of the stars in your children can be a wonderful gift to them, encouraging them to study the math and science classes that lead to academic excellence. Even better, there's a widely used telescope design that can be made at home as a craft project. This is a broad overview of the project--it's not the same as a step-by-step manual.

The Optics
- You'll need two mirrors, one of them convex and one of them flat. The convex mirror is your collection mirror and can be from 5 to about 12 inches in diameter, with a focal point that's some distance away (read the exact specifications of the mirror you buy). The second mirror is your focal mirror. It will be situated in the barrel of the telescope at a 45-degree angle. It will focus the image up to the eyepiece and will probably be an inch or so in diameter. While some people call a Dobsonian telescope a refractor, because it's usually enclosed with a dust plate on the front of the instrument, it's actually a reflector design. There are several places where you can get both mirrors online. If you're truly brave, there are classes on grinding your own mirror blanks and aluminizing them; this is a lot of meticulous work, but it's not difficult. It is the only way you're going to get a mirror larger than about 12 inches for the collection mirror.
The Tube
- You'll need a tube that's sufficiently broad in diameter to hold the aperture mirror in one end. Concrete forming tubes are good for this and can be found for low prices (sometimes even for free). The main manufacturer of this tube is a company called Sonoco, and in the "build-your-own-telescope" community, they're sometimes called Sonotubes. Once you get the tube, you're going to need to spraypaint the inside of it with a deep black paint. You want something that absorbs all the light that's not going to the collection mirror.
Mounting the Optics in the Tube
- Your collection mirror is going to be put at one end of the telescope; the best way to fit it is to mount it on something very stable and to put a rubber gasket around the outer edge, then put the matching gasket on the inside of the tube. You can also find commercial matching thread gaskets that will do the same job. The smaller focal mirror will be mounted on something called a spider, which will suspend it rigidly at the right distance from the collection mirror. Remember that the focal mirror has to be tilted so that it reflects the spot of the beam up where the eyepiece will be. A lot of people who build Dobsonian telescopes put a glass plate on the "far" end of the telescope to keep dust out of the optics.
The Eyepiece Mount
- Your eyepiece mount will be a pair of nested steel tubes, with a pair of rollers. The rollers will be used to raise or lower the interior tube, and the interior diameter of the interior tube should be 11/4 inch to fit most eyepieces. You can buy commercial eyepieces from Meade or Celestron or Edmund Scientific, or scavenge ones off of old binoculars--7x50 binoculars are a good candidate for eyepieces.
The Telescope Mount
- You need to make an upright that will hold the telescope and allow you to change its angle going up and down. This should be made out of heavy-duty plywood with two ring-bearing joints on either side. Next, you want to mount the azimuth box on a lazy Susan or a phonograph turntable, so you can rotate it around in the horizontal axis.

Astronomy