A FEW WEEKS ago one of the largest and most carefully made amateur astronomical observatories went into operation several miles north of Amherst, Va. The event happened to coincide with the 50th anniversary of telescope making as a popular avocation. The observatory and its telescope are the handiwork of John Wikswo, a chemist, and his son John, Jr., a senior at the University of Virginia who is majoring in physics.

The installation is remarkable in many respects. For example, the instrument embodies more scaled-down features of the 200-inch Hale telescope on Palomar Mountain than I have encountered heretofore. The two-story observatory building, made of masonry, has a motor-driven dome of reinforced fiber glass. The building includes a photographic darkroom, a machine shop and a study. All the functional elements are controlled electrically, including a system of remote dials that continuously monitor the telescope's position as it tracks an object across the sky.

Wikswo tended to look down his nose at telescope making. The deceptively simple appearance of a reflecting telescope rarely challenges the inexperienced eye. After much urging by Wyld, however, Wikswo reluctantly agreed to try his hand at a six-inch instrument He spent almost a year on the project. Most amateurs would have finished it in 60 days. The difference was that Wikswo's telescope rivaled the excellence of a finely made watch. Wyld took one look through it and contrived a scheme for binding Wikswo forever to his avocation. Although Wyld was young and in perfect health at the time, he provided in his will for the purchase of two specially cast 16-inch disks of Pyrex as a bequest to Wikswo. In 1952 the disks were delivered. Wyld knew his man. The astonished Wikswo has been working on the telescope ever since. Recently he invited me to inspect the results.

To reach the observatory you turn left off Route 29 about 10 miles north of Amherst and follow the narrow road that winds northwest through the Blue Ridge mountains. Somewhere between the villages of Piney River and the Forks of Buffalo the road enters an oval valley. Atop a large knoll that rises in the valley you spot the dome of the observatory under a sky that is usually crystal clear. A breath of the fresh air almost justified the trip. "We were against pollution in these parts," Wikswo explained, "long before the idea caught on elsewhere. My neighbors on the crests of these mountains learned from skilled ancestors the art of firing a whiskey still without creating a telltale plume of smoke. That's one reason I'm here." Wikswo explained another of his reasons as we made our way along a footpath to the observatory. A vein of titanium ore runs for seven miles through a neighboring mountain. He heads a group of chemists that develops methods for processing the mineral.

The door of the observatory opens onto a landing from which a spiral staircase leads to the observing floor above and the darkroom below [see illustration at right]. The cylindrical wall is made up of curved cement blocks. The overlapping dome is formed by two dozen curved trusses of fir overlain by a sandwich of Masonite and fiber-glass cloth cemented with polyester resin. The weight of the dome is carried by a base ring of laminated wood; the ring has flanged wheels that roll on a circular track of steel. The track was bent from bar stock two inches wide and half an inch thick. It is anchored to the rim of the concrete wall.

The telescope is supported at the center of the dome by a concrete pier that extends down through both floors to bedrock and is insulated against mechanical vibrations. You can jump on either floor without vibrating the instrument. The dome is rotated by frictional contact between the laminated base ring and a pneumatic tire on a motor-driven wheel. Access to the sky is through a pair of weatherproof shutters in the dome; they open to form a slit one yard wide from the horizon to the zenith. The shutters, which were built in an old farmhouse that came with the property, are also made of Masonite and fiber- glass. They are operated with a hand wheel through a system of cables. This is one of the few features of the observatory that Wikswo has not mechanized.

The fork that supports the tube of the telescope and rotates the instrument in right ascension, or celestial longitude, is of the welded box-girder type stiffened with internal partitions [see illustration at left]. The fork includes bolting plates an inch thick for attaching the shaft and the housings of the declination bearings. Although the instrument weighs more than 600 pounds, it moves easily in any direction.

The tube is rotated in right ascension by a synchronous motor that operates at 1,800 revolutions per minute in combination with a gear system that includes a differential mechanism. A direct-current motor coupled to the differential is used for turning the tube to any part of the sky. The gearing in both declination (north-south direction) and right ascension includes selsyn (self-synchronous) transmitters that couple electrically to companion receivers operating dials for monitoring the position of the instrument. When the telescope is tracking a star automatically, a selsyn motor geared to the polar shaft communicates the rotation synchronously to a companion motor coupled to one input shaft of a differential in the dial mechanism that indicates right ascension. Concurrently rotation in the opposite sense is fed into the second input shaft of this differential by a clock motor that keeps sidereal, or star, time. The two inputs cancel. Accordingly the dial remains fixed and indicates the unchanging position of the telescope in right ascension.

When the telescope is stopped, the clock takes over. The dial now advances at the sidereal rate, thus indicating the telescope's changing position with respect to the sky. To aim the instrument at a particular object, Wikswo consults a star chart for the celestial coordinates. By operating push buttons he directs the instrument onto the target as indicated by the dials.

The tube is of the open-cage type and consists of five rings of cast aluminum weighing 20 pounds each. They are supported axially by a series of tubular spacers held in compression by six steel tie rods [see illustration at right].

The tube rotates in declination on hollow shafts of cast iron fixed to ribbed cast-aluminum plates that form part of the tube assembly. The shafts turn in ball bearings that are carried by the fork. A 4-1/2 inch steel shaft, salvaged from an old ore mill, was fitted with ball bearings for rotating the fork in right ascension. The perforated objective mirror floats on a nine-point suspension mechanism inside a cell of cast aluminum that closes the back of the tube. Patterns for all castings were made in Wikswo's basement workshop.

A paraboloidal mirror is the heart of all reflecting telescopes. An instrument can be no better than its mirror, however much care the maker may lavish on the mounting. Wikswo kept careful notes as he made his 16-inch mirror, and from them he prepared the following account:

"The first thing you need when you undertake the construction of a telescope larger than about 10 inches is a machine for grinding and polishing glass. It is simple to make a small concave mirror by sandwiching a slurry of Carborundum grit between matching disks of glass and pushing the upper disk back and forth across the lower one by hand. During a portion of each stroke the upper disk overhangs the lower one; maximum pressure between the two develops in the central portion of the upper disk and around the edge of the lower one. The grit abrades the glass in proportion to the pressure Hence the upper disk becomes concave; it eventually serves as the mirror. The lower disk, which becomes convex, functions as the grinding tool and is discarded when the job is finished.

"Stroking the upper disk back and forth by hand is possible, although not necessarily easy, with disks up to about 10 inches in diameter. The thickness of the disks must be at least a sixth of their diameter to prevent the mirror from flexing and thus grinding out of true. The thickness of a 16-inch Pyrex casting as it comes from the factory is more than three inches, and the glass weighs about 60 pounds. You do not push a disk of this size by hand.

"The machine I made was based on the design described in Scientific American [see "The Amateur Astronomer," by Albert G. Ingalls; SCIENTIFIC AMERICAN, May, 1950]. Essentially it consists of a rigid turntable 18 inches in diameter, mounted on the upper end of a motor-driven shaft, and a rigid bar that oscillates lengthwise above the turntable. The bar is driven by a crank that is coupled to the motor by a train of gears. Both the relative and the absolute speed of the crank and turntable can be altered.

"The disk of glass to be ground and polished is mounted face up on the turntable. Glass tools of appropriate diameter are pushed back and forth across the mirror by the oscillating bar. The grinding can be done with a slurry of grit, as in the hand technique. The machine lends itself to other grinding techniques, however, and even to other uses, as I learned from experience. For example, I adapted it for machining the mirror cell and other aluminum castings used for the tube.

"The Pyrex disk I chose for the mirror was slightly oval and wedge-shaped. Grinding it to within .005 inch of a true cylinder with parallel faces required 100 pounds of No. 54 Carborundum grit, which has particles about the size of the grains in granulated sugar. The rough glass was prepared for grinding by cementing a disk of 3/4-inch plywood 15 inches in diameter to the bottom of the casting with hot pitch. The wood disk was centered on the steel turntable of the machine and screwed in place. The top face of the glass was first ground parallel to the face of the turntable by a drill press provided with a tool made from an eight-inch length of heavy aluminum pipe eight inches in diameter. The tool was chucked in the press and rotated by a drill shank that projected from the center of a metal disk, which closed one end of the pipe. The open end of the pipe, which functioned as the cutting edge, was supported parallel to the top of the turntable. A slurry of bentonite clay and Carborundum was applied to the working edge of the tool. A teaspoon of clay mixed with three pounds of wet Carborundum forms an adhesive mud that speeds the rate of abrasion.

"Most amateurs grind the edge of a glass disk simply by wrapping a strap of sheet metal around the disk, rotating the disk and applying abrasive slurry. The technique wastes glass. The edge of my disk was ground true with abrasive slurry applied to a rotating brass cylinder, three inches in diameter, that was also chucked in the drill press. The turntable was rotated by hand until all high spots had been ground from the edge. The glass was then turned by the motor at 2-1/2 revolutions per minute as successively finer grades of abrasive were applied to the tool in preparation for the polishing operation. The brass cylinder rotated 100 revolutions per minute. The finely ground edge was polished by wrapping around the disk a metal band lined with felt charged with rouge.

"After the edge reached full polish I detached the wood disk from the glass with a hammer blow that fractured the pitch. The turntable was covered with a disk of sheet plastic. The glass was turned over, centered on the plastic rough side up and clamped in place by three equally spaced blocks of maple bolted to the edge of the turntable. The unfinished side was ground parallel to the bottom with the eight-inch tool.

"The interior of the glass could be examined for defects by looking through the polished edge. I selected for the working face of the mirror the side that contained the fewest bubbles. This side was then ground to a concave spherical figure of 160-inch radius, twice the desired 80-inch focal length. To generate the curve, I canted the drill press by shimming up the outer end of the base to an angle that formed the 160-inch radius when the eight-inch tool abraded the glass. At this stage some 300 hours of work had been invested in the mirror.

"The telescope was designed to provide access to the focal plane at any of three locations: the distant end of the tube, called the Newtonian focus; through the hollow declination axis, or coudé focus, and at the rear of the objective mirror, known as the Cassegrainian focus. In the first two of these arrangements the converging rays are reflected outside the tube at right angles by plane mirrors. In the Cassegrainian arrangement they are reflected backward by a small convex mirror through a perforation in the center of the objective mirror.

"The required perforation was trepanned in the disk with abrasive applied to a three-inch pipe chucked in the drill press. The cut was made halfway through the glass from both sides to avoid chipping the surfaces. After the inner surface of the perforation had been trued (by lapping the hole with fine abrasive applied with a cylindrical tool of brass) the glass plug was replaced and cemented with dental plaster as a mechanical support for the surface during the final grinding, polishing and figuring of the paraboloid. In this step I made use of a clever trick devised by Fred Cowan of the Lick Observatory optical shop. The glass plug was covered with a closely wound layer of No. 22 steel wire before the cement was applied. The plug was removed easily when the mirror was finished by unwinding the wire.

"The coarse abrasive used for generating the curve left deep pits in the surface. These were ground away, in preparation for polishing the glass, with successively finer grades of abrasive applied with a tool in the form of a disk that was pushed back and forth by the oscillating arm of the machine. The tool moved at a rate of 1.5 inches per second. The mirror simultaneously rotated at a speed of 2-1/2 revolutions per minute. The diameter of the tool was 13.3 inches, five-sixths of the diameter of the mirror. The tool was a ribbed disk of steel faced with an orthogonal mosaic of two-inch glass squares 3/8 inch thick cemented to the steel with epoxy. The edges of all the squares were beveled to an angle of 45 degrees by hand with a fine Carborundum stone and water. When shifting from each of the coarser grades of abrasive to a finer grade, I thoroughly washed and dried the mirror and the tool. The tool was also repainted each time, including the spaces between the glass squares. This precaution minimized the possibility that a grain of coarse grit might be carried into the succeeding stage of fine grinding and scar the glass. No scratches appeared. The mirror was ready for its final polish after 51 hours of fine grinding.

"The polishing tool was made by placing gummed paper dams around each of the glass facets and filling the cavities with tempered black pitch. Thirty-five hours of polishing with rouge eliminated all traces of the fine grinding and brought the surface to a polished spherical figure. The curve was then deepened into a paraboloid by altering the length of the strokes and displacing the tool laterally as necessary. Various optical tests were made during this stage for monitoring the changing shape of the curve. Work was stopped when tests indicated that the surface was within a millionth of an inch (approximately a twentieth of a wavelength of light) of the desired paraboloid. The complete job of converting the rough casting into the finished mirror required 440 hours of work.

"The telescope cost just a little less than $500. The largest single item w $150 for foundry castings. The grinding machine, abrasives and evaporated aluminum coating of the mirror amounted to another $150, not counting $60 for express charges. Another $200 was spent on the shop and darkroom. The observatory cost $1,500.

"Occasionally I am asked why I made the observatory of cement blocks. Masonry structures leave something to be desired because they function as heat sinks that create convection currents in the cool night air and thus degrade seeing. I took this disadvantage into account when I decided on masonry, for protection against speeding bullets. High-powered rifles are legal for hunting deer in this part of the country. Not all our city visitors have the skill of my neighbors at handling a gun."

Wikswo's telescope performs beautifully, as you can see for yourself if you wish. Wikswo holds open house occasionally at night. Visitors who write for reservations are welcome. The address John Wikswo, Route 4, Box 224-A, Amherst, Va. 24521.

THE CIRCUIT diagram of the power supply for the tunable laser using organic dye, described in this department in February, shows a connection between the primary and secondary windings of the oscilloscope transformer. The connection invites the destruction of one of the diodes and should be omitted, as several readers have pointed out.

Bibliography

AMATEUR TELESCOPE MAKING (BOOK THREE). Edited by Albert G. Ingalls. Scientific American, Inc., 1953.

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