The 48-inch Schmidt is now beginning its first work as a systematic scout for the 200-inch. During the next four years it will photograph, area by adjacent area, in both red light and blue, the whole of the sky that is visible from its latitude- three fourths of the entire area of the heavens. The result will be an album of a thousand pairs of photographic prints, costing $2,000 a copy, which will be used by other observatories.

As this program swings along, at the rate of one exposure an hour when the skies are clear, new discoveries almost ; surely will be made as by-products: data on variable stars, on novae, on clusters, and unexpected finds: a new asteroid has been found already. What, then, is the principle of the Schmidt, and where did it get its name?

Bernhard Schmidt, the son of Swedish and German parents, was born in 1879 on Nargen, an Estonian island between Estonia and Finland. After studying optics in Sweden he moved to the vicinity of Jena in Germany, where he lived a carefree life for 25 years, occasionally making telescope mirrors to sell to amateur and professional astronomers. He ground them not only by hand but with only one hand; he had lost his right hand when he was a boy. He never married. Dr. Paul C. Hodges, who has investigated his life, says that Schmidt's wants were confined to simple lodgings. food, cigars, cognac and freedom from regimentation. He would undertake no more mirror making than would barely satisfy these few needs, and refused positions with the large optical manufacturers.

In 1926, however, he was induced by astronomers of the Hamburg Observatory in Bergedorf to join the staff as a "voluntary colleague," an arrangement that gave him freedom to roam the woods and talk to himself. In regimented Germany this loose kind of arrangement might have cost the director of the Observatory his job but, as Dr. Hodges states, "instead it gave the world the Schmidt camera." Schmidt announced its discovery in 1932. He died in 1985 but not before the significance of the basic principle he originated had been recognized by science. Even greater realization of its importance has come since his death.

This is the problem that Schmidt solved: If only a spherical mirror could be used in a reflecting telescope, the telescope would be free from the blurring aberration called coma. Unfortunately a spherical mirror suffers from another cause of blurring called spherical aberration. To remove this spherical aberration the sphere is "corrected," that is, slightly deepened toward the center to make it a paraboloid. This correction in turn introduces coma, which, for critical uses such as star photography, blurs all images not in the center of the field. Thus in picking up one bundle the second bundle is dropped.

Bernhard Schmidt discovered a way to carry both bundles. He analyzed the problem thus: A correction equivalent to a paraboloid could be made on a separate plate of glass in contact with the original spherical mirror. From this, however, there would be no gain, as many others had realized after having the same analytical thought. Schmidt's stroke of genius was to think one more thought-and, of course, the right one. He removed the plate from contact with the spherical mirror and shifted it several feet to the center of curvature of the sphere. Here both kinds of aberration were eliminated at once, making possible a sharp focus over a broad area on the photographic plate. By easy hindsight, most optical workers have noted how simple the Schmidt principle is, and have kicked themselves for not discovering it first.

Cameras like the 200-inch that have paraboloidal mirrors can photograph, blur-free, areas of the sky only about one sixth of a degree in angular diameter, represented approximately by holding a pin at arm's length with its head facing the eye. The 48-inch Schmidt at Palomar can photograph areas six degrees in angular diameter, represented by holding three fingers at arm's length against the sky. The ratio between the two areas is about one to 900. This gain was Schmidt's great gift to astronomy.

The six-degree angular diameter of the area photographed by the 48-inch Schmidt is almost the same as that achieved by a Schmidt camera built by the amateur astronomer John F. Gregory of t3825 Bainbridge Road, Cleveland Heights 18, Ohio. This does not mean that Gregory's 5-inch Schmidt, shown in Figure 3, equals the great Palomar Schmidt in all respects. It has very much less power to penetrate distant space.

In Figure 1 is a photograph of the Double Cluster in Perseus and surrounding stars made by Gregory's Schmidt. The original of this photograph, a little round film only 1 5/8 inches in diameter, was shown, together with photographs of the Gregory instrument, to Dr. Henry Paul of Norwich, N. Y., an advanced amateur authority on the Schmidt. The Perseus Cluster photograph evoked praises not often given to or justified by similar attempts: "Very good in many many respects." The images of individual stars are small and round, showing no aberrations, clear out to the edges of the film.

GREGORY is a senior student in mechanical engineering at the Case Institute of Technology in Cleveland. Ten years ago, at the age of 12, he attended a lecture by J. J. Nassau, head of the department of astronomy at Case. A year later, with the help of the book Amateur Telescope Making, he had completed a beginner's reflecting telescope. Wisely, he waited to acquire more years and experience before tackling a Schmidt.

The lightweight tripod shown in the illustration is temporary, being found too shaky on breezy nights. Bolted to the top of the tripod is a block of oak. To this the electric driving motor and the driving-speed reduction gearbox are bolted below; the mounting and tube are bolted above. The main casting of the mounting, as well as the fork and the gearbox, are of aluminum. Gregory "patterned," cast and machined them as a special project while taking required courses in these subjects at Case. The sloping polar axis is mounted in a 5/8-inch radial ball bearing at its lower (south) end. Another bearing of 1 1/4-inch diameter at the upper end of this axis absorbs the end thrust due to the camera's weight.

The drive that keeps the camera in motion to compensate for the earth's rotation consists of a Warren C5M synchronous motor rated at 12 watts and costing $13. It is attached to the lower end of the gearbox and rotates at four revolutions per minute. The drive passes through a differential like the one shown on page 323 of Amateur Telescope Making-Advanced (to permit the use of a flexible cable for hand guiding during photographic exposures), then to 18and 15-tooth meshing spur gears, next to 61- and 75-tooth gears. All are Boston Gear Works stock gears. All the shafts are mounted on ball bearings. Where it emerges from the gearbox the drive is connected with the camera by a vertical shaft with jaw coupling which permits disconnection of the gearbox from the unit. The next gear, above the oak board, is a worm and 60-tooth worm gear K pressed directly on the main worm shaft of the polar axis. The worm wheel has 96 teeth.

The tube of the telescope was made by a local sheet-metal worker who rolled up and butt-welded a sheet of .050-inch thick 3S1/2H aluminum that cost $1.70 at a local warehouse of the Aluminum Company of America (general office: Pittsburgh, Pa.).

"The primary mirror was made from a 6-inch plate glass blank, ground and figured to a sphere of 14-inch focal length," Gregory says. "Thus if the 5-inch stop in the focal plane was removed the focal ratio would be 2 1/3, but with it the working focal ratio is f3. The 5-inch stop provides for equal illumination over an area as large as a 4-inch stop would provide if placed at the mirror's center of curvature. Because the curve is so deep, the figure tended to remain spherical during polishing and little trouble was experienced in figuring.

"The glass for the correcting plate was from an Air Corps filter. I thought this glass would be flat enough, but I had to grind it flat with No. 600 Carborundum."

For the correcting plate Gregory chose the type of curve based on k = 1. This gives a curve that is worked from a plane surface with a neutral or deepest zone .707 of the distance from the center of the plate to the edge. "Having drawn the curve," he states, "I cemented half-inch-square glasses, cut from a two-inch-square slide cover glass, to a sponge rubber kneeling pad, with most of the squares at the radius of the thinnest zone, tapering off gradually toward the center and very rapidly toward the edge. After 15 minutes' grinding with finishing emery, and an hour's polishing with cerium oxide on HCF backed with sponge rubber, the lens was ready to test.

"The test described by Lower in Amateur Telescope Making-Advanced, page 412, was found to be satisfactory, but I used a Ronchi grating instead of a slit, making it by photographing a coarse grating on film to give about 30 lines per inch. Placing this film at the focus (within half an inch) and viewing the mirror through the lens from a distance of 10 feet or so, I discovered that all except the outer inch was satisfactory. Regrinding, polishing and testing from 30 feet finally gave a much-longed-for appearance of parallel lines.

"The longitudinal focusing mechanism I evolved is shown in the drawing. It is the easiest arrangement I know of for 'squaring-on' the spider and finding the focus precisely, micrometrically, yet quickly. I threaded a short stud one inch in diameter with 24 threads per inch and press-fitted it to the spider. A second stud, movable, was threaded with 20 threads per inch. A rotating brass collar was made in two pieces, one half threaded with 24 threads per inch and the other with 20 threads per inch, and the two halves joined with screws. This collar moves the movable stud longitudinally at a very gradual, controllable rate. As the collar is turned one revolution it moves on the fixed stud 1/24 inch, but at the same time off the movable stud 1/20 inch. Since the movable stud is constrained against rotation by a concealed pin shown in the drawing, it moves only 1/20 inch-1/24 inch, or a delicate 1/120 inch, with respect to the stationary stud.

"In adjusting the focus, which is delicate on a Schmidt, where everything is critically sensitive, a series of exposures are taken without moving the plateholder on the stud. After each exposure the collar is rotated, the camera also being moved in declination so that the next exposure will not fall on the first. In this manner one can focus and square-on after about three test plates.

"The plateholder fits over and remains on the movable stud by friction, and consists of a camera filter adapter, a flat metal back-plate, a black paper disk, the film held flat, and a crown glass planoconvex field flattener lens ( radius of convex curve 5 inches), all in the stated sequence and all pressed into mutual contact by the retaining ring. The field flattener for the convex field characteristic of the Schmidt camera was resorted to after unsuccessful attempts to keep the film from humping out of contact with the conventional convex base. Except for flare due to reflection around bright stars, it has worked very well, especially since it was coated.

"On nights with no haze or smoke I can expose for an hour before sky illumination fogs the film.

"The guide telescope is a 2-inch refractor and is too small: it is unfortunately very hard to guide on faint stars with it.

"For visual observation I have a diagonal and a 16-mm orthoscopic ocular held on a metal plate that fits over the side of the tube. Because of the steep cone of light, images are good over no more than a three-degree diameter field, but most Schmidts are not used visually at all."

ONE more source of puzzling scratches on optical surfaces has been tracked down. Carl E. Wells of Roseville, Calif., reports that D. A. McLaren of Hunter's Point, Calif., was long baffled by unaccountable scratches, as are many other workers. His inspiration was to examine the chamfer at the edge with a microscope. He was astonished to note the manner in which this part was fractured. Acting on the hypothesis that these tiny fractures break off during polishing and scratch the glass, he threw away the Carborundum stone used to make the chamfer and substituted a piece of glass and loose Carborundum grains No. 600. This stratagem finally put an end to his scratches.

Ironically the use of a Carborundum stone for chamfering is recommended in Amateur Telescope Making (pages 286 and 296) for the prevention of scratches! It prevents some scratches in a manner that apparently causes others. The history of the telescope making art from its beginning has been one of additional learning.

OUR SUN, by Donald H. Menzel, assistant director of the Harvard College Observatory, covers the current status of solar research from all its major angles: an easy approach; the basis of astrophysics; solar chemistry; the problems of sunspots, or solar cyclones; fine details of the solar surface; solar prominences as phenomena of explosiveness; the corona mystery; atomic energy and the sun's interior; solar eclipses, and the sun as a source of energy for man's use. This coverage is as broad and as deep as that of a textbook with the happy difference that, not being dragged down by the consciousness that he is writing a textbook for use in the discipline and to be criticized as such by others, an author can write more readably and the publisher can print more attractively. The result, in the present instance, combines the values of both qualities. Dr. Menzel has done much writing for lay readers and his style is as straightforward and unstilted as that of a lecturer before an intimate audience. The subjects dealt with are technical, the book is by no means light. It deals with solid subjects at the upper middle level of understanding.

Suppliers and Organizations

The Society for Amateur Scientists
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Phone: 1-877-527-0382 voice/fax

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

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