How To Make A Telescope Cheaply
by R. Goff, 2A
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There are many people, Im sure, who have enjoyed the magnificent view at their disposal on the top of Harrow Hill.. Maybe a lucky person has a good pair of binoculars of average magnification (about 25X). But I'm not really concerned with binoculars; Im dealing with something maybe a bit cumbersome and heavy; but, in ninety nine cases out of one hundred much more powerful, THE TELESCOPE.

Of course, the easy way of obtaining a telescope is to buy one. But for the individual who hasnt got so much cash to throw around, the best thing is to make one. I can assure you, it needs hardly any mathematical knowledge.

There are two basic types of telescope; the 'reflector' and the refractor. Refractors are a bit of a problem. Lenses have to be ground (quite a difficult operation). They also have to have matching faces for the objective Lens. So I'm leaving out the refractor.

This leaves us with the reflecting telescope, the basis of which is the concave mirror. To get about making a telescope, then, you need a concave mirror. Taking economy as the main objective, I suggest that the mirror should be home-made. You can grind the mirror if you've got the time, but if you haven't this is what to do. Get a clock glass of about 6 in diameter (if you want a 6 reflector), and get it silvered on the convex side. (Id have it done at the shop myself.) It will cost about eight shillings; quite reasonable when you remember that a brand new ground; polished mirror will cost you at least 7 if it is 6 in diameter). Next, the tube is needed. Don't bother making that. They're cheap enough. But before you buy the tube a word of warning. FIRST MEASURE THE FOCAL LENGTH OF THE MIRROR. That's most important. A tube 3ft. long isn't much good for a mirror of 50" focal length (the length from the centre of the mirror to the point of focus). Assuming our 6 mirror has an f.1 or 60 I'd advise a tube about 60 long.

We've now got the mirror and tube. Erect the mirror at one end of the tube. The light, rays will come down the tube; they will be reflected and 'focussed' at a point 60 down the tube. Where do we go from here?

Obviously one cant look straight down the tube at the mirror; a ray deflecting device is require. That's quite simple; all we need is a small plane mirror about 1 square facing the centre of the mirror, INCLINED AT AN ANGLE OF 45 degrees PLACED JUST IN FRONT OF THE FOCAL POINT to deflect the rays at right angles to the main mirror's principal axis (imaginary line from centre of mirror to the focal point).

Since we don't want the "twice reflected" rays to remain in the tube we need a hole in the top of the tube to allow the rays to be magnified by an EYEPIECE, which incidentally will be in a holder attached to a short metal rod dropping into the tube, and supporting the secondary mirror, But theres just one snag. The image will be upside down. Astronomically this doesn't matter, but for terrestrial viewing an erector lens is needed. Used one way round it halves the magnification; used the other way found, it doubles it. Cost? 15/- approximately. One last word - to determine the power of the telescope, divide f.1 of eyepiece into f.1 of mirror, and good viewing!

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