IT HAS LONG BEEN THE policy of this department to publish descriptions of two categories of telescopes built by amateurs: simple ones that almost anyone can make, and elaborate ones that all may at least examine, perhaps envy, and hope someday to emulate or excel. As a matter of fact, so many amateurs have first-class equipment and the skills to use it that a fairly large proportion of amateur telescopes do fall in the elaborate class. But it is still true that a small, unpretentious telescope, especially if it is well thought out and built, is as worthy a member of the telescope community as those made by the more fortunately equipped. Probably there are many beautiful little instruments that have never been described in print because of the modesty of their builders. Such will take precedence over merely showy instruments.

The 10-inch reflecting telescope described below falls in both the elegant and excellent categories. It has been described by those familiar with it, neighbors of its builder, as "a first-class, ultra-perfectionist job, better than professional in quality." These are not idle boasts. A professional could not sell enough ultra-perfectionist telescopes to break even; he must make compromises in order to set a price that the average buyer can afford. John S. Stewart of 1007 South Seventh St., St. Charles, Ill., an engineer and amateur telescope maker, was the designer and builder of this instrument, which is chiefly described in Roger Hayward's drawings in Figures 1 and 2.

The skeleton tube consists of 16 lengths of tubular steel welded into two zigzag units attached to cast-bronze rings at the ends and to a one-piece, square cast-bronze saddle box at the balancing point. With this design the whole assembly is highly rigid. A section of circular aluminum tube is attached to the upper ring of the skeleton tube and rotated to place the observer's eye in the most convenient position. Two sections of tube are available for this purpose; the shorter is used with an f/5 mirror to view faint objects such as nebulae and Milky Way star dust, and the second much longer one goes with an f/10 mirror for observing extended objects such as planets. The amount of light collected by the two 10-inch mirrors is the same, but the short-focus mirror concentrates it by reflecting it to a narrower circle, while the long-focus mirror magnifies it more by spreading it over a wider circle, which can be afforded because the planets give more light.

The excellently designed fork is cast in duralumin and has a 14-inch clearance between the tines. The tapered polar-axis shaft attached to it rotates in a plain bronze taper bearing at the top and in a tapered roller end-thrust bearing at the bottom. Pinned on this axis shaft with a taper pin is a hub and web, forming the fixed plate of a clutch. Surrounding this plate at its periphery is a 100-tooth worm gear, free to rotate, like a dog's collar around its neck, when the capstan screw that holds the clutch pressure plate against it is temporarily loosened. For convenience any one of the four screws will lock the clutch, which will also act as a slip clutch if the screw is only moderately tightened. The tube can then be pushed around by hand, yet the clutch will drive when the hand lets go. The right-ascension slip ring can be rotated at any time, being held in position by friction.

The welded channel steel frame of the mounting is attached to a four-foot cube of concrete flush with the ground. A streamlined case covers the telescope's vital organs as high as the fork.

Within the square gearbox, submerged in one gallon of lubricating oil, is the precise mechanism for driving the telescope as the earth turns, so that the observer need not be continually distracted by the necessity of moving it by hand. This drive also permits making celestial photographs with time exposures if desired. Outside of the box are two electric motors, the driving motor and a smaller correction motor. The first is a 1/20 horsepower Bodine motor of the synchronous capacitator type running from power lines at 1,800 revolutions per minute. It turns a 120-tooth worm gear and shaft within the box by means of a flexible coupling. At the nearer end of the same shaft in the drawing (passing for the moment over six gears pertaining to the correction motor's functioning) is a 51-tooth 20-pitch gear. This meshes with a 79-tooth spur gear on a countershaft. The drive next passes outside the gearbox by way of a shaft and worm on a countershaft. This countershaft carries another worm that drives the worm gear on the polar axis. This combination, 51-79-41-98, is the fifth in the list of gear ratios for converting standard to sidereal time given by Alan E. Gee on page 322 of Amateur Telescope Making-Advanced, and it leaves a driving error of only .02-second per day.

To correct the driving error and larger transient errors due to unpredictable variations in atmospheric refraction and other causes, intervention in the drive is obtained by means of a second, separate motor. This is started, stopped or reversed at will by a push button held K in the observer's hand at the end of a flexible cable. This occasional interference is artfully accomplished without altering the speed of the main driving K motor even slightly and, indeed, without its knowledge of the impertinence. It is achieved by a differential gearing unit working on the planetary principle. This is the system we temporarily skipped over in the last paragraph.

In this unit the small central sun gear is pinned to the input shaft and rotates with it at all times, at its rate of 15 revolutions per minute. The sun gear meshes with three planetary gears, so these also constantly rotate on their pinions, which are fixed in the side of the large worm gear. Since this worm gear remains stationary most of the time, the planetaries merely idle around without effect within the internal gear that encircles them. But now if the observer, finding the telescope is gradually falling a little behind the stars, pushes the "add" button, the worm on the correction motor shaft will rotate and turn the large worm gear a little. Since it has a reduction gear of its own built into it, the speed of its input shaft is but 10 revolutions per minute.

This new motion, introduced without disturbing the main or driving motor of the telescope, is transmitted through the pinions, planetaries, internal gear and hub plate and will be added to the rotational speed of the input shaft. If, on the other hand, the telescope creeps ahead of the stars, the correction motor is reversed and kept rotating until the error is removed. With the gearing combinations and speeds shown in the drawings, these increases or decreases in speed are obtained at the gentle rate of only one tenth of a revolution per minute. (The . 10 revolutions per minute of the motor has been reduced this much by the 100tooth worm gear.)

A NEW BOOK entitled La Construction du Telescope D'Amateur, by Jean Texereau, is so thoroughly practical that if it were in English it would be an event in the telescope-making world and would be given an extended review in this department. Its author has stated in personal letters that he began as an amateur in 1988 with The Amateur's Telescope and Amateur Telescope Making and its companion volume as his guides in mirror making. He is now a technical collaborator (optician) at the Paris Observatory and has made a 24-inch Cassegrainian telescope for the Meudon Observatory. He has illustrated the 136 pages of his book with numerous drawings of the highly explanatory kind that the American illustrator Roger Hayward made for John Strong's Procedures in Experimental Physics. Half of the book is devoted to mirror making, the remainder to plane-parallel diagonal making, eyepiece making, mountings, accessories and collimation. The publisher is the Société Astronomique de France, 28, rue Serpente, Paris (VI^e), France, and the price is 650 francs, about $1.90 (the amount to remit is currently ascertainable at any post-office money-order window).

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-401-823-7800

Internet: http://www.sas.org/

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