The telescope is shown as a visual Gregorian in the drawing. The eyepiece is at the lower end of the tube. At the opposite end a small concave secondary mirror reflects to the eyepiece the rays it receives from the large primary at the bottom. (This is shown more clearly at the upper left of the drawing on page 78.) The effect of these reflections, made at angles determined by the calculated curvature of the paraboloidal primary mirror and the ellipsoidal secondary, together with the effect of the added double folding, is to shorten the telescope greatly. The tube of this one is 56 inches long, with an effective focal length of 205 inches; a simple telescope with the same magnification would be about 17 feet long.

The gains of the Gregorian, however, are not obtained without certain losses; this is nearly always the case in optical design, which is by nature a compromise. Shaping the curve on the small secondary mirror is a fussy, delicate task. Lining up the mirrors of a compound telescope is another delicate job. The observer may pay with a wry neck and sagging knees for the whim of looking in the direction of the object. The image is erect, but astronomers are accustomed to inverted images and find erect ones abnormal. (This feature nonetheless makes the instrument useful as a terrestrial telescope.) The Gregorian's virtues include a higher magnification than the Cassegrainian, another type of compound telescope. In any case, the beginner should make two or three simple telescopes before tackling a compound such as this.

In the second of the four uses of the Whitney telescope, a film carrier is substituted for the eyepiece at the bottom of the tube, and photographs are made. In the third use, as a visual Newtonian, the Gregorian secondary mirror remains in the tube but is nonfunctional, and the Newtonian secondary and eyepiece carrier unit is inserted and quickly latched to the side. Its secondary mirror, a flat diagonal, intercepts the rays from the primary and reflects them to a position outside the tube where they can enter the eye, as shown at the upper right in the drawing below. The f/3.45 low-magnification Newtonian, best adapted for viewing faint objects, may also be used on the planets when the earth's atmosphere is so unsteady that high magnification causes greater loss in sharpness than gain in size.

Finally the Newtonian focus may be used photographically, not by substituting a film holder for the eyepiece, but by inserting it directly at the center of the tube and taking the photograph at the prime focus. This eliminates one reflection.

The Whitney telescope has many fine mechanical and optical features. The mirror cell at the lower end of the tube is made from an automobile brake drum; its central opening is covered with a metal plate to which the eyepiece and assembly are attached. When a photograph is to be taken, the eyepiece is taken out without removing the assembly, and a metal box with a 4-by-5 cut-film holder is attached by side clamps. A Packard-type shutter inside the cell is operated by a rubber air bulb.

A photograph of the moon taken by Whitney looked so fine that it was sent by this department to Dr. Henry Paul, an advanced amateur who specializes in astronomical photography. He commented, "This is the best moon picture by an amateur I have ever seen." (He modestly omitted his own.)

When the sun is to be photographed, a tube with a Polaroid filter hinged at its top is inserted through the hole in the primary mirror and screwed into the backplate, the Polaroid filter is flipped into the path of the light rays, and the primary mirror is stopped down to one-eighth area. This tube is long enough to serve another function: it shields the eyepiece from the direct light of the sky. Otherwise this sky-flooding glare would render the Gregorian virtually useless as a daytime terrestrial telescope.

The instrument is mounted on a deep broad concrete pier by means of a five-inch 45-degree pipe elbow. Inside this elbow, running on a propeller-pitch type bearing, is the combined polar-axis and declination-axis unit. This was machined from a 1928 Chevrolet differential housing. Fine adjustment is imparted to the two-inch declination shaft by hand screws on either side of the mounting. These work through a linkage to a sawed-off connecting rod bolted to the shaft.

The guide mechanism, rebuilt from a war-surplus bomb-train intervalometer has a knob that affords manual control of the driving rate for guiding the telescope during photography. Guiding is done with a 3 1/2-inch elbow refractor of 40-inch focal length attached to the tube but not shown in the drawing. The rectangular object shown near the center of the tube contains a battery for illuminating the reticle of the guide telescope. The drive motor is a synchronous General Electric phonograph turntable motor, and the gear train is built up of spur gears.

The 12-inch primary mirror was made from a very ancient disk of green porthole glass two inches thick, obtained from a ship chandler. Today plate glass is not made thicker than 1 1/2 inches.

Theoretically Pyrex is three times as good a mirror material as plate, since its expansion coefficient is only one third as great, but arguments could be adduced that in practice might narrow this difference considerably in most cases. During the excavation of the concavity, which is almost a quarter of an inch deep, the loss of glass thickness by grinding was held to a minimum by the use of sub-diameter tools. The mirror curve was excavated to about two-thirds the ultimate depth on top of an 8-inch tool. The hole was then bored from the face to within 1/2-inch of the back. Next, the same tool was used on top until the curve was nearly excavated. Then the hole was deepened to within 3/16-inch of the back and the slot was packed with wicking and paraffined over. Polishing and figuring were done face up, by hand, with the mirror on a turntable driven at six revolutions per minute. A lap having two-thirds diameter was used for smoothing, and laps of 3 1/2-, 1 1/2- and 3/4 inch diameter were used for treating zones and figuring. Finally the central plug was knocked out.

The telescope is housed in a circular dome of sheet aluminum bolted to shallow U-channel ribs of iron. The 10-foot dome diameter proved to be none too big and, while the 34-inch dome slot is wide enough, it is none too wide. The dome rotates rather rapidly at the rate of two revolutions per minute, driven by a 28-volt electric-clutch-operated reversible war-surplus motor, reduced in speed by a system of V-belts controlled from the pier.

THIS DEPARTMENT is now illustrated with drawings instead of photographs. As Russell Porter often pointed out with a chuckle of satisfaction, the camera cannot be inserted in the places where an artist can put his imaginary point of view, nor can it make all-revealing combination cut-away illustrations. The composite drawing above was made by Roger Hayward from 21 photographs, only two of which could have been reproduced in the same space. The 21 photographs, mainly of disassembled parts of the telescope, together show only a few minor details not combined in the single drawing; yet only one part of the drawing, the mechanism immediately on top of the pier, approaches a direct copy of any single photograph.

The recombining is done after the photographs or rough sketches and the written description have been assimilated. With the understanding thus gained, the artist can safely alter the point of view or manner of presentation as much as he wishes, especially if he is himself an amateur telescope maker and mechanic, as Hayward is. For example the single drawing at the bottom alone contains the elements of four photographs, plus a little that even these did not reveal. The reader need have no fear of loss of integrity in transmission.

A NEW WAY OF FOLDING the optics of a telescope into zigzag form to keep the length within reason, as in the Gregorian and Cassegrainian, has been suggested by Daniel E. McGuire, a Pittsburgh professional optician. McGuire began as an amateur, and retains the keen interest in amateur optics of all who add professional to amateur status. (This combination, impossible in athletics, is harmless in optics. ) He writes:

"I had the lucky opportunity to look through the most perfect telescope that I have ever used. It was one of the off-axis reflectors of Norbert J. Schell of Beaver Falls, Pa. For the first time in my life I have seen in great detail the belts of Jupiter, yet the aperture of the telescope was only six inches. I couldn't get away from the impression that I was looking through a much larger telescope. The high power that goes with its f/21 ratio probably contributes to that impression."

Schell also conceived and-with T. G. Beede, who made the off-axis mirror- constructed the "crisscross unobstructed" instrument shown in the next drawing. Here the optics are folded triply into a short tube, and light baffles prevent direct sky-flooding at the eyepiece.

MeGuire's present proposal was inspired by the fact that Schell recommends, as others have, a closed tube as an added refinement for his unobstructed telescopes. McGuire writes: "If one goes so far as to use a window to close the tube, why not make the window in the form of an achromatic lens of very long focus? Then the 'off-axis' mirrors could be plane mirrors, which are much easier to make. The length of tube usual for a refractor having a ratio of f/15 would accommodate a ratio of f/45. For any given magnification color fringes would be reduced to a third of what they are in the typical refractor."

McGuire continues: "The idea of making a refractor with an unusually long focus to reduce the effect of the secondary spectrum came to my mind when considering the difficulty of making off-axis mirrors. The refractor is much easier to make, although the greater focal length brings about mounting problems. In Telescopes and Accessories, by Dimitroff and Baker, page 27, it is stated that the focal length of an achromatic objective with unobjectionable color aberration is given with sufficient accuracy by the formula f = 5D². By this standard a 3-inch objective having a focal ratio of f/15 has unobjectionable color. A 6-inch at f/30 and a 9-inch at f/45 would have the same amount of color at a given magnification per inch of aperture, but the telescope would be much too long in the conventional straight-line design.

"At the expense of using a wider tube and including auxiliary plane mirrors, the tube length can still be kept within convenient proportions. Two types of 'achromatic brachytes' are shown in the drawing. Mounting possibilities would include the Springfield arrangement with the eyepiece on the polar axis and the coudé Cassegrainian (Amateur Telescope Making, page 452) with eyepiece near the declination axis."

IN a frequently quoted article in the 1 February, l938, issue of The Journal of the British Astronomical Association, H. E. Dall of England described experiments on "diffraction effects due to axial obstructions in telescopes." Using various sizes of central obstruction, Dall made photographs of artificial planets consisting of a uniformly illuminated aperture overlaid with wires corresponding to lines 42, 84 and 168 miles wide on the Martian disk. Under these controlled laboratory conditions, which he described in a private communication as an attempt "to find out how much effect was due to the central obstruction and how much to air-current trouble in open tubes," he demonstrated that the effect of diffraction is to reduce contrast as well as definition but that "if the central obstruction does not exceed one fifth of the diameter of the aperture no serious loss of detail contrast occurs. Above this proportion of obstruction a noticeable falling off of crispness of detail results, accompanied by a growth of spurious detail and diffraction haloes." To avoid losses even less than serious, Dall still recommends the method of minimizing obstruction that he described on page 584 of Amateur Telescope Making-Advanced: a tiny prism just a little inside primary focus feeding the rays to a good erecting system. "By this means," he says, "I brought the secondary obstruction down to less than 6 per cent of diameter or .36 per cent of area, and got an erect image too."

"Of course," Dall adds, "for real simplicity and merit the oldest of the lot, the Herschelian, takes a lot of beating, as I am always trying to drive home. The decentering error can be reduce in several ways to less than the Rayleigh tolerance, provided the focus is not mad too short, say, less than f/8. Off-axis cylinders or lens tilts will do it."

Suppliers and Organizations

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