The design of this great reflector has been under way for three years. A 120inch, 8,500-pound ribbed Pyrex mirror blank has now been received and temporarily stored. The first thing to be built will be the observatory dome, since the mirror is to be ground, polished and figured within it, in a temporary optical shop located between the telescope foundation and the dome wall. This will make possible both horizontal and vertical testing of the mirror during polishing and figuring.

A staff of engineers under the direction of Senior Engineer Wilbert W. Baustian, formerly of Caltech, has carried out the design of the telescope. A massive 85-ton fork mounting will give access to the entire heavens at the latitude of Mount Hamilton. The tube, similar in principle to that of the 200inch, will be a welded structure of the open truss type, built of steel plate and tubing with square mid-section and round ends. It will, however, have much longer relative proportions than the 200inch, the mirror having a focal ratio of f/5 instead of the Palomar telescope's f/3.3.

The 120-inch has been designed and is being built to profit from Caltech's experience with the 200-inch. The Pasadena institution has generously made these data completely available to the Lick workers. On the advice of the builders of the 200-inch, the plan until a year ago was to make the 120-inch mirror of 16-inch-thick solid glass. At that time the users of the 200-inch disk still regarded its ribbed construction as experimental. The 200-inch had to be ribbed because the transmission of heat through a disk is retarded in proportion to the square of its thickness. The necessary equalization of temperature throughout a solid 200-inch disk 24 inches thick would be an insoluble problem, for the temperature of the atmosphere is always changing and the disk's temperature would never catch up with it. With its ribbed construction the 200-inch mirror is only four inches thick in a temperature sense. It was held, however, that this kind of hindrance to temperature equalization would not be too critical in the 16-inch-thick solid disk recommended for the Lick telescope.

After the 200-inch ribbed mirror proved successful, however, the Lick designers bought the 120-inch ribbed Pyrex blank from the California Institute of Technology for $50,000. This blank, which has recently been delivered, was cast at Corning, N. Y., in 1933 for conversion into an optical flat for testing the 200-inch mirror. Other methods of testing were substituted, and the blank had stood unused in the optical shop at Pasadena for many years. Lick also bought the large grinding and polishing machine that was to have converted it into a flat.

The 120-inch blank is nearly perfect, remarkably homogeneous and uniform. On its front part, which is four inches thick, the curve of the new mirror will be excavated a maximum of 1 1/2 inches. The disk will then be perforated with a hole eight inches in diameter to afford passage of light rays from the secondary mirror back to the Cassegrainian focus.

Difficulties that have long delayed the final completion of the 200-inch mirror were caused mainly by the fact that at no time during polishing and figuring could it be tested in the same generally horizontal position in which it is used in the telescope. For testing, it always had to be turned on edge. An irregularity in curvature-a high edge zone-was purposely not removed before the mirror was placed in the telescope, because theory indicated that when it was in horizontal position the bulge would sag out. When the mirror was placed in the telescope this theory proved erroneous. Yet testing the 200-inch mirror in a horizontal position would have required the temporary construction above it of a 125-foot tower, with insulation to prevent a change of temperature in the optical shop beneath it. At Lick the "tower" will be the 94-foot-high observatory dome itself, and this is why the dome must be built before work is begun on the mirror and mounting. Finishing the mirror within the dome will also permit its easy insertion in the mounting as often as required during that process.

Perhaps the most arresting feature of the 120-inch telescope, as drawn from the original designs by Roger Hayward (himself an amateur telescope maker), is the huge and stocky fork with its angular outlines. This was explained by the late Russell W. Porter: "The fork form, which disturbs me, is due to the fact that it was impossible to compute the stresses it curved tines were used, it had to be computed with straight sections."

The Lick project will require about four or five years for completion after actual construction is begun. Although the telescope is not the largest, it has at least as much claim to interest among telescope makers and users as the 200-inch telescope. The 120-inch probably will permit exploration of the universe to a distance of 900 million light-years, and there is more than enough research to be done inside that limit of distance.

IN 1944 the Russian, Maksutov, announced in the Journal of the Optical Society of America his invention and patenting of a photographic and visual type of telescope that has become known as the "Maksutov," more familiarly the "Mak." Of the existing types of highly corrected photographic telescopes, the Mak most closely resembles the Schmidt. It has a spherical primary mirror at the bottom of its tube and a correcting lens at the top; but the correcting lens, unlike that of the Schmidt, is thick and deeply concave. In his detailed 15-page article Maksutov described its principle and suggested several interesting variations on its central theme-the Herschelian Mak, the Cassegrainian Mak, the Gregorian Mak, the brachyte Mak, and others.

Norbert J. Schell of Beaver Falls, Pa., studied the Maksutov article, selected the simplest type-the Newtonian Mak-and at this magazine's invitation wrote articles, published in October and December, 1944, in which he gave all the design data necessary for making a Newtonian Mak. At the same time this department organized a buyers' club of advanced amateurs to reduce the cost of molding and casting the thick blanks of glass necessary for making the meniscus correcting lens (concavity 2/3-inch deep) at the skyward end of the Mak. The Corning Glass Works built a temporary mold and made 24 of these special blanks of crown glass having a refractive index 1.517 and a dispersion 64.5, each 5.2 inches in diameter and 1 3/4 inches thick. Then the mold was broken up and all the blanks were sold.

So far as is known only two Maksutovs were completed among this group. In 1945 Arthur DeVany of Des Moines, Iowa,. made one for a comet seeker. In October, 1947, G. Camilli of Pittsfield, Mass., described in SCIENTIFIC AMERICAN the one he made and said of it: "The performance of the Mak well repays all the work put into it; the definition is much superior to that of a simple reflector."

Two other Maksutovs were also nearly finished. One could not be brought to full perfection and finally trailed off into a state of innocuous desuetude. The other, shown in the illustration on the opposite page, has been virtually completed by Dave Broadhead of Wellsville, N.Y.

This is a Mak of the inventor's "simplest and most fundamental system," which is basically photographic but has a support for a Newtonian diagonal for visual use. The latter can be substituted for the film holder merely by reaching through the hand hole in the side of the tube and replacing it with the film-holder support shown in the insert sketch. These two supports may be exchanged by means of a wing nut that attaches them to the center of the corrector lens; there is no spider.

"Visually," Broadhead says, "the Mak" gives a wonderful view and all that the eye can take; the exit pupil is a little larger than the pupil of the eye. The stars are sharp to the very edges of the field, a fact that psychologically increases the apparent field diameter."

Circumstances forced Broadhead temporarily to set this telescope aside before it could be completed by the addition of a drive, but it is far from a dead duck. Meantime it is hoped that a description of it will reactivate those whose Mak meniscus corrector lens blanks are still unfinished, perhaps because they are waiting to see how other Maks make out. Or perhaps they will agree to transfer these blanks to other owners who will convert them into the vital parts of Maksutovs. Such aspirants cannot now obtain blanks from the original source of supply; there is no mold.

The drawing contains most of the description of the Broadhead Maksutov. The cored aluminum fork, cast thin from a furnace in Broadhead's cellar shop (which is also equipped with lathe shaper and the fundamental machine tools), is a splendid featherweight example of skilled workmanship.

In his article in the October, 1944, SCIENTIFIC AMERICAN, Schell pointed out the chief advantage to the builder Of the Maksutov over the otherwise similar Schmidt: the Schmidt corrector plate has a shallow and irregular curve that is difficult to make, while the meniscus corrector lens of the Mak has two spherical curves that are much less difficult. Like the Schmidt, the Mak enjoys the advantage of a closed tube, with its accompanying suppression of internal air currents that damage good seeing; and like the Schmidt the Mak system is aplanatic, having neither spherical aberration nor coma.

In the specifications worked out by Schell and followed by Broadhead and K the others, the primary mirror at the bottom of the tube is made from a standard S-inch Pyrex telescope blank to a radius of curvature of 65.856 inches. The corrector lens, .800-inch thick, has an internal radius of curvature of 12.688 inches, and an external radius 12.224 inches. Primary and lens are separated 43.128 inches. As these precise measurements suggest, the Mak is scarcely a job for a novice. Letters from those who have worked on Maks, largely discussing suitable tests-Ronchi and interference, mainly-are available on loan to new workers.

In July, 1946, C. J. Tenukest, R. Shaefer and H. Pinnock of New South Wales, Australia, described in The Journal of the British Astronomical Association a 6-inch Maksutov built from data in the Maksutov article in the Journal of the Optical Society of America and in SCIENTIFIC AMERICAN. For the corrector lens they used crown glass of index 1.51694 instead of 1.517, and also of a different thickness, and were thus forced to recalculate the specifications. Their Mak was a success. "Remarkably small and sharp images of stars were obtained," they stated, "free from coma and color. Such images are hardly possible with reflectors even on the most perfect night. The image of Jupiter was as sharp as if viewed through a first-grade refractor, yet the bluish halo visible around the disk with even the best glass was entirely absent. The image was crisp and colorless.

Slabs of glass at least 1 5/8 inches thick and having characteristics close enough to that specified by Maksutov (5163641) may be available from American manufacturers, though not in the approximate meniscus form cast by Corning.

In his article Maksutov stated that he invented his telescope in August, 1941, and that it was patented in the U.S.S.R. On November 3, 1941. After the war when Holland emerged from its wartime confinement, the Dutch optical designer A. Bouwers published in English his book Achievements in Optics (Elsevier Publishing Co., New York) which described a telescope essentially identical with the Maksutov. With the secret cooperation of the Dutch patent office, Bouwers had patented this type of telescope July 7, 1941, or four months before Maksutov. It is now said that K. Penning, a German, applied for a German patent on the same principle March 6, 1941.

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