A Homemade Telescope

With our homemade telescope you can get an enhanced view of the nighttime celestial show.


| May/June 1985



homemade telescope - diagonal mirror

The diagonal mirror is glued to a section of 3/4" dowel and hung from a 1/8" threaded rod.


LAWRENCE A. JACKSON

"Space — the final frontier ...."

Those words, spoken by Scar Trek's Captain Kirk many years ago, still hold true today. There are billions of stars, planets, moons, and comets (of which Halley's is only one) out there for us to explore. Of course, during this generation at least, few of us will be lucky enough to actually venture into space. But, on starry nights, we can look up and wonder.

A naked eye peering into the sky can divine a few of the universe's secrets, but with a telescope your vision can reach deep into space for answers. For a novice stargazer, though, buying a telescope is a big step. To get a top-notch new scope, you'd have to invest hundreds of dollars ... and what's to say that you (or your aspiring child astronomer) won't lose interest after a few nights?

The solution — one that will require more personal and less monetary investment — is to build a homemade telescope. With tools no more sophisticated than a power saw, a router, and a drill — and some care in construction — you can produce a quality 3-3/8" Newtonian reflector telescope that is comparable to commercial scopes costing three or four times as much!

Telescope Optics

Though it's possible to make the optical parts of a reflecting telescope, the process is laborious and exacting, and the savings aren't all that great. For example, I ordered a curved 3 3/8" main, or primary, mirror with a focal length of 42", and a matching diagonal mirror to reflect light rays into a 31/32"-diameter eyepiece with a 15 mm focal length. The three pieces were purchased from the A. Jaegers Jr. Company for $55.50 (plus shipping and handling).

The Tube

Once you've gotten the two mirrors and the eyepiece, you can begin constructing the telescope tube. Cut the four sides out of a quarter sheet of 3/8" A-C exterior plywood, guided by the dimensions in Fig 2 of our Telescope Assembly Diagram. Then paint the C-grade sides flat black (a brush application works better than aerosol), and tack the tube together, painted side in, with wood glue and 4d finish nails spaced every 6". What you'll get is, well, an ugly rectangular box. Just remember, the beauty of a telescope comes not from what it looks like but what it looks at ... and how well it does so!

The Cell

Set the tube aside, and begin work on the holder for the main mirror (this component is called a cell by astronomers). Though it would be convenient to just glue the mirror to the bottom end panel of the tube, that approach won't do. The light from the stars and planets travels many millions of miles to get to your telescope, and if the mirrors aren't lined up perfectly, the scope's performance will be substandard at best. You must provide a way to adjust the mirrors after they've been mounted.

The Assembly Diagram shows you the dimensions for building a primary cell to fit the A. Jaegars No. 6B1429 primary mirror. To fit other mirrors, adjust the routed-cavity dimensions so the mirror will be flush with the cell's rim and have about 1/16 inch of play within the rim's diameter. The goal is for the mirror to be able to slip smoothly into the cell without rattling around. When you finish with the router, dill and countersink a 1/8" hole in the center of the cell, and cut the plywood to the diameter shown in the diagram.

Next, cut the square 3/8" plywood bottom for the tube, drill a 1/8" hole in the center of the board, and bore three 3/16" holes around the central one at 120° intervals on a radius of about 1 1/8". These 3/16" holes will form an equilateral triangle with sides of about 2". From the A-grade side of the plywood, twist a 3/16" x 1 1/2" thumbscrew into each of these holes until the screw sticks out 1/2".

To connect the cell to the bottom panel, insert a 1/8" x 2" flathead machine screw through the countersunk side of the central hole in the cell, and spin a nut down on the back of the cell to hold the screw in place. Slip this fastener through the central hole in the tube bottom, and snug the back of the cell up against the three 3/16" thumbscrews by tightening a wing nut onto the 1/8" screw. Don't overtighten the wing nut. Spin it on just far enough to hold the cell against the points of the thumbscrews.

When the time comes to restrain the mirror in the cell, don't glue it, as this would prevent it from expanding and contracting with changes in temperature. Instead, use three No. 4 x 1/2" wood screws and 1/8"-thick plastic tabs, spaced at 120° intervals around the perimeter of the cavity.

Eyepiece and Diagonal Mirror

The rays of light coming from the main mirror are reflected into the eyepiece (at a right angle) by the diagonal mirror. The eyepiece, in turn, converges the rays further, effecting additional magnification and producing an image for you to view. Therefore, these components must be accurately positioned if your telescope is to produce clear images.

Measure 40" from the bottom of the tube (not including the 3/8" bottom board), and mark the location of the center of the eyepiece by making a line all the way across the top panel of the tube. Move one inch farther up the tube, and mark the position of the diagonal mount. Why 40" when the focal length of the main mirror is 42"? The reflective surface of the main mirror is 1-1/4" above the bottom of the tube (1 /2" of screw and 3/4" of plywood), so the actual distance from the main mirror to the center of the diagonal (directly below the eyepiece) is 38 3/4". It's 2 1/2" from the center of the diagonal mirror to the outside of the tube wall, which leaves 3/4" for the eyepiece mount. Thus, once the 3/4"-thick board is added for the eyepiece mount, the focus of the main mirror will be exactly at the outer edge of the eyepiece hole.





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