"When the Mexican Government contracted with the Perkin-Elmer Corporation for a large Schmidt camera, the aperture was specified as 24". The very outermost zones of a corrector plate, however, do not come to proper curve by the treatment which is best for the inner zones. In the ease of a 24" Schmidt previously made by this company for the Harvard College Observatory, the plate was not oversized, and consequently the edge zones delayed the final figure. For the Mexican instrument, therefore, Halley Mogey, Chief Optician, requested a larger blank, so that the 24" clear aperture could be obtained in the least time. The grinding and edging operations were directed by T. J. La Lime, head of the grinding department.

"Although the curve on this corrector plate is imperceptible to the eye (the focal ratio is f/3.5), the difference between it and a perfectly flat plate is an amount of glass as big as a lump of sugar (about a quarter of a cubic inch) all removed by polishing alone.

"Both the knife-edge and the Ronchi grating were used in the test set-up, which involved auto-collimation from a flat mirror and the use of a small periscope for looking at the image without obscuring much of the beam. When the mirror and plate were completed, no errors at all could be seen under this test over at least a 24 1/2" aperture."

The three shop photographs, Figures 1, 2, and 3, were furnished at our request by Richard Perkin. They were taken by the former amateur telescope maker, Robert E. Cox, who a year or two ago "went professional" with the Perkin-Elmer Corporation; another former amateur employed there being Daniel E. McGuire who similarly went professional several years ago. Amateurs are now in so many professional optical shops that this department finds it almost impossible to draw a line between the two. There actually is no such line; it has been washed out. Most of the professionals were amateurs at one time.

Figure 1 shows the 31" Schmidt mirror disk in the last stage of emery before polishing. The machine' itself will also interest our readers. So will chief optician Halley Mogey, who stands beside it. (Some day this department may publish a series of personality articles on prominent people in professional optics.) Your scribe believes Halley Mogey's name never is mentioned without the inevitable after-question, "Was he named for Halley's Comet?" Halley Mogey really was named for Halley's Comet. What was more logical-his father, a telescope maker and the two arrivals coinciding? The boy grew up in his father's shop at Plainfield, N. J. (now run by another son, William Mogey, assisted by, Messrs. Brown, LoJas, and Grosswendt, all former amateurs). There he gained the "feel" of glass and abrasive. Then he went off to college and gained theory, and returned equipped on both sides, practiced and theoretical, to make maximum application of the mind-and-hand team. When Schmidts came along, a few years ago, he went to the Mt. Wilson optical shops to learn special Schmidt technique; and there some of his own. In Figure 1 he is seen in shop clothes. A good optician, no manner how much theory he has in he head, never is too lofty to get into working pants and use his hands.

Figure 2 shows the same 31" disk fine-ground, with T. J. La Lime, head of the grinding department, measuring the radius of curvature with a spherometer.

In Figure 3, Halley Mogey is hand-touching an outer zone on the 26" corrector plate of the Schmidt camera. Some years ago amateurs argued whether the tradition that the old-time professionals used their hands as a "rubber" (as non-telescope making persons styled it) for local touching, was true or just a story. While some argued, others tried it, and it worked. Since then it has been amply proved by other evidences that the old-timers did use this method; the popular tradition was quite correct Besides, the method is not in any way remarkable; simply, instead of a small tool of pitch, a small tool of fine leather is used-that is, the hand or whatever part of the hand is the user's own pet method. In Figure 3 you see this method in the flesh. This photograph, now published, will be useful to show to occasional doubters about hand touching among professionals.

NO clearer, simpler, more direct explanation of the working principle of the Schmidt camera has been seen by this department than one found in a booklet describing the newly added equipment at the Warner and Swasey Observatory of the Case School of Applied Science at Cleveland. The drawings from the booklet are reproduced in Figure 4, and here is the explanation:

"In 1931 Bernhart Schmidt, an optical worker at the Hamburg Observatory in Bergedorf, published an account of a reflecting telescope free from nearly all the inherent defects common to such instruments. This was accomplished by introducing a thin lens in front of a spherical mirror.

"His reasoning was clear and logical, in spite of the fact that he avoided all mathematics. He ended his article by stating: 'The method of producing the lens is assumed.' In other words, he was not going to reveal the secret, a characteristic common to optical workers, particularly of the past.

"It seems that the first Schmidt telescope was completed in the summer of 1930, when Schmidt and a friend amused themselves by reading the epitaphs on the tombstones in a nearby cemetery. A remarkably detailed photograph of a windmill a mile and a quarter away, made with his first telescope during a moonless night, is now of historic interest.

"In 1936, Dr. R. Schorr of Hamburg Observatory finally disclosed the secret of how Schmidt, who had died in 1935, had produced the surface of the all-important lens. Schmidt placed a plane-parallel disk of glass on the open end of a circular cylinder of nearly the same diameter and evacuated the cylinder until the desired bending of the glass disk was secured. Then he polished the glass surface to a perfect plane. After allowing the air to re-fill the cylinder, the plane polished surface took the desired form of the lens.

"The Schmidt-type telescope employs a spherical mirror instead of the usual parabolic mirror of the reflector. Parallel rays incident upon a spherical mirror do not come to a focus at the same point. The rays striking the center of the mirror come to a focus at a point F, midway between the mirror and its center of curvature; while rays striking the outer zones come to focus somewhat nearer to the mirror.

"If, however, we make the light striking this outer zone slightly divergent, all the rays can be brought to a focus at F. This is accomplished by placing a thin lens in front of the mirror and at its center of curvature. The shape of such a lens is shown m the second diagram.

"This results in the combination in the third diagram.

"The same results may be accomplished by introducing at R a lens with a shape shown m the fourth diagram.

"In this ease, the outer rays are less divergent and the center rays slightly convergent. The resulting combination, with the focus now a little closer to the mirror, is shown in the final diagram.

"If the parallel rays are directed to the telescope from an angle different from the one shown in the above figures, they will again come to an exact focus. This is due to the fact that the mirror is spherical and has no axis, and the lens is at the center of curvature of the mirror. The optical system thus remains exactly the same as before.

"The bundles of parallel rays striking the mirror from all angles will thus be focused on a photographic plate bent into a slightly spherical surface. In practice it is-easy to bend ordinary flat photographic plates into the necessary spherical form.

"Stated in another way-rays of light entering the telescope from stars widely separated in the sky will be brought to an exact focus; a condition not realized in the usual reflecting telescope. Thus a large area of the sky may be photographed at one time.

"Since the lens employed is relatively thin, it produces an instrument which is free from the color defects associated with refracting telescopes. Also, since the focal ratio of the telescope can be made small without sacrificing other desirable properties, the instrument is 'fast' in the same sense as the word is used with respect to cameras.

"The wide-angle field, the achromatic properties, and the speed, are the three. principal advantages of the Schmidt telescope."

The new Schmidt camera at the Warner and Swasey Observatory has a 36", Pyrex mirror of 14' radius of curvature, with a 24" lens of Vita-glass, .3" thick. Maximum departure from a plane is .0005" and the optical work was done by C. A. Lundin of the Warner and Swasey Co., Cleveland, Ohio.

BEFORE the beginner has picked up from here and there a modicum of general background about telescope principles he is almost sure to run up at least one blind alley in pursuit of a beautiful illusion. Your scribe, for example, in 1924, when quite innocent of telescoptical background, saw a picture of a group of reflectors made by the Telescope Makers of Springfield, Vermont. One of them was short, chubby, compact, unlike the others. Obviously that one would be much better, hence the one to make. Now it happens that a short chubby telescope (short focal ratio) is an ideal one for the tyro not to make, because the curve of the mirror is much more difficult when deep-more advanced work. Russell Porter explained this fact and your scribe desisted.

On the opposite extreme are telescope having great length-long focal ratio-and when the tyro discovers that these give high magnification he is inclined to plump for one as his first job. These, however, are a headache in other ways. As John Pierce puts it: "Made a 4" of 90" focal length with very poor results so far u comfort in observing goes-hard to point-diagonal large and field necessarily sr"4 since the large-lens eyepieces are not available."

Commenting on a tyro's proposal to make a "high-magnification" telescope with focal ratio 40, Alan R. Kirkham made apposite observations in an old letter dug up: by your scribe from the lower, or pre-Cambrian, strata of his desk (had a house cleaning):

"Focal ratios of longer than f/20, which itself is extreme, cannot be recommended because they do not give full fields wit any but eyepieces whose magnifying power is above most practicable limits. The focal image of the Moon in a telescope of aperture and focal ratio-of 40 is big-about 2.088" in diameter-it is true. However compare the size of the field lens of a 1" eyepiece with this and see how miserably small the field actually seen will be. In order to get down within the magnifying power universally agreed to be best for seeing, about 16 diameters to the inch, perhaps 10 to 20 diameters depending various factors, we require a 2 1/2" eyepiece with such a telescope. Good eyepieces are easily designed for a useful field of 40°, but the ability to give this apparent ran depends on the field lens being large enough to receive all the rays of 40° magnification. The condition is met with in all eyepiece of about l 1/4" focus, down. A 1 1/2" eyepiece needs a field lens 1 1/8" in diameter, a 2" eyepiece one of 1 1/2" diameter, and a 2 1/2" should have a field lens about 2" in diameter. These 'out' sizes are hard or impossible to find-it's about like asking for a size 22 shoe at a shoe store. In a nutshell, we get only a narrow field, caused by the telescope itself having wrong proportions. Moreover, these eyepieces would have great spherical aberration. They also require several inches of rack and pinion work for focusing from the longest to the shortest of them, and require diagonals of too great size. They also suffer from curvature of field.

"If that is not enough, there are other reasons why the existing range of eyepieces became more or less standard-for it didn't just 'happen.' The long focal ratio takes a very long and awkward tube, and means trouble in keeping the optical elements in alignment. Besides, try figuring an f/40 mirror-I did. Just like looking at the head of a pin. Diffraction effects are very great, and the mirror even seems to move around with the motion of eye. In focograms, such mirrors are surrounded by a diffraction edge half the radius of the mirror itself. Finally, the image is poorly illuminated."

Suppliers and Organizations

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