THE CLEAREST COLOR PHOTOGRAPHS of Saturn and Mars ever to reach the attention of this department appear below. Another, of Jupiter, adorns the cover of this issue. All are the work of Robert B. Leighton, a nuclear physicist at the California Institute of Technology who insists that in the field of astronomy he is still an amateur. These remarkable pictures are the initial products of an electromechanical guiding system which Leighton designed and built. He made the photographs with a larger telescope than most amateurs ever see, much less use-the 60-inch reflector at the Mount Wilson Observatory.

Leighton is one of those fellows who is happiest when he has at least a half dozen balls in the air at once. During recent years, while becoming a young parent, he has built a modern ranch-style house with his own hands, carried a steady teaching schedule at Cal Tech, made basic contributions in the field of nuclear physics and squeezed out sufficient free time for many hours on his hobby of amateur astronomy. Just now he is finishing the construction of a rooftop observatory on his house, with a 16inch Cassegrainian telescope controlled by a precision electronic drive.

Several years ago, while working with a cloud chamber atop Mount Wilson as part of a cosmic ray study, Leighton became restless between observations and soon found himself up to his elbows in one of astronomy's most stubborn problems: how to circumvent poor "seeing" of stars due to the turbulence of the earth's atmosphere. The swirling air above the observatory causes the image o f a star or planet to twinkle and shift around on the photographic plate so that it registers as a smudge instead of a sharply defined point or disk. Making the telescope larger and more powerful only aggravates the smudging. Tourists who visit the big observatories, hopeful of a glimpse of life on Mars, often express chagrin when told that the surface of a planet or of the moon appears no clearer when viewed through the 200inch Hale telescope than through a little 10- or 12-inch glass. "Why, then," they demand, "do you waste millions building these huge white elephants?"

This department recently asked Leighton to answer that question, and, while he was at it, to explain how he had made his magnificently sharp pictures of the planets.

"It is a common belief," he writes, "that very large telescopes, such as the 100-inch and 200-inch reflectors of the Mount Wilson and Palomar Observatories should show fine detail of the moon and planets because the resolving power of a perfectly figured telescope lens, for objects situated at a great distance, is supposed to improve as the diameter of the objective lens is increased.

"According to the so-called Rayleigh criterion, the resolving power of a telescope is expressed numerically by dividing 4.5 by the diameter of the objective in inches. The quotient gives the smallest angular separation, in seconds of arc, at which two equally bright point sources of light can be distinguished.

Any pair separated by a smaller angle, will merge because of the wave nature of light and be seen as a single point. Hence if the performance of the huge telescopes were limited only by their optical quality, they would indeed give breathtaking views of the planets.

"Although the big instruments are virtually perfect in their optical and mechanical construction, and have surpassed their expected performance in the applications for which they were designed, there is very little likelihood that they will ever show planetary detail to the limit of their theoretical capabilities. In fact, there is good reason to believe that the best planetary and lunar photographs will be made with telescopes of but [30 to 40 inches aperture.

"An observer at the eyepiece of a relatively small telescope can see millions of tiny craterlets and other structures less than a thousand feet wide on the moon, but no lunar photograph has yet pictured detail less than about a mile in extent. Again, the ring system of Saturn has never been adequately photographed. Most photographs show only the two main rings and the largest division, whereas the faint crape ring is very clear visually in small telescopes, and at least two other divisions are recognized. The canals of Mars may be a third example. Their failure to appear on any of the many thousands of photographs that have been made of this planet is the cause of a long-standing controversy regarding their existence. Many qualified observers have reported seeing them. Others, apparently equally qualified, see not a trace of them. Even a single convincing photograph could settle the question of their existence.

"These examples, particularly that of the lunar craters, clearly establish that a wide gap exists between well-substantiated visual observation and the corresponding photographic results. The fundamental cause of this discrepancy is to be found not in any lack of optical perfection of the telescope itself, but rather in the optical unsteadiness of the earth's atmosphere, which is brought about by thermal nonuniformities always present throughout it. This, coupled with the need for several seconds' exposure, leads to relatively poor photographic resolution, no matter how large or small a telescope is used.

"The degree of optical steadiness of the atmosphere is called the 'seeing.' One effect of the turbulence is visible to the naked eye as the scintillation or 'twinkling' of the stars. As the thermally inhomogeneous regions move past the observer's line of sight, they act upon the light rays passing through them, thereby producing a constantly changing deviation and phase shift. Neighboring rays interfere with one another and cause the observed color and brightness changes. In times of good seeing the atmosphere is relatively calm and thermally uniform and there is little or no naked-eye stellar scintillation. During bad seeing, on the contrary, the atmosphere is quite nonuniform thermally and a large degree of scintillation is visible.

"The atmospheric turbulence that leads to poor seeing arises from many causes. It may be created by local warm objects (such as motors, vacuum tubes or observers near the telescope itself), by a difference in temperature between the telescope tube and the surrounding air, or by nearby chimneys or factories which emit heat. More basically, the inhomogeneities are caused by large-scale convection currents which accompany cloud formation and thunderstorms, or by turbulence between atmospheric layers having different temperatures and wind velocities.

"The character of the seeing can be viewed telescopically in considerable detail by observing an out-of-focus image of a bright star.

The pattern you see resembles the bands and spots of sunlight on the bottom of a slightly agitated pool of water. They are in constant motion Slow-moving patterns with sharp boundaries generally signify nearby heat sources, and these can often be tracked down by careful observation. Fast-moving patterns can usually be seen sweeping across the objective in one or more directions; these are caused by winds somewhere in the atmosphere.

"The effect of the seeing upon the quality of an image formed by a telescope depends upon the 'cell size' of the seeing. This refers to the size of the region over which the air temperature is . sufficiently uniform so that a parallel light beam passing through such a region to the telescope is negligibly distorted. The part of the objective that receives such a beam forms a perfect image. If the cell size is substantially smaller than the telescope aperture, the objective will encompass several such cells, with the result that a number of separate images are formed by the telescope. These then combine to form a blurred image on which fine detail cannot be resolved. If only a few such cells cover the aperture, the separate images may be individually visible. Each star or other object is split into a small cluster. This is often the case with fine detail such as the craters of the moon or the Jovian satellites.

"At the other extreme the seeing cell-size may be much larger than the telescope aperture, so that the entire objective acts as a unit and the resultant image is clear and sharp. But the image will move irregularly about some average position. These irregular excursions are often as large as one or two seconds of arc, which is several per cent of the angular diameter of Mars or Jupiter. Under given conditions of seeing it is clearly disadvantageous to use an aperture larger than the seeing cell-size. This aperture will yield a brighter image, though it will show less detail.

"What is the best size of telescope, then, for visual observation? It ought to be large enough to take advantage of the best seeing (i.e., largest cell-size) that is reasonably likely to occur. The maximum size thus depends upon the geographical location, for at each location there is a certain distribution of seeing conditions throughout the year, and on each night there is a corresponding maximum useful aperture for visual observation. On most nights, even at a favorable location, this will be less than three or four inches. On many it may be as large as 10 or 12 inches. But apertures as large as 50 or 60 inches very seldom can be used with maximum effect. The greatest telescopes, such as the 100-inch and 200-inch reflectors, will encounter seeing conditions fully matching their apertures only once in many years. Indeed, no astronomer who has used the 200-inch Hale telescope has yet reported star images less than about three tenths of a second of arc in diameter. This size corresponds to the theoretical resolving power of a 15-inch telescope! Obviously a visual observer gains no advantage at the eyepiece of the huge telescopes.

"If we now consider the photographic situation, a new element enters the problem. This is the requirement that a sufficient exposure time be provided to yield a satisfactory photographic image. Because of this the advantage of a smaller aperture disappears, since the fainter image corresponding to the smaller aperture requires a longer exposure and will therefore move about more on the film, yielding a blurred image. Thus for photographic purposes it is almost immaterial whether a large or small aperture is used, so long as it is at least as large as the seeing conditions will permit for visual observation.

"It should now be clear why direct vision has proved superior to photography for the observation of lunar and planetary detail. For visual observation of a sufficiently bright object, it is of no great importance that the image be steady, so long as it is sharply defined, because the eye is able to follow the irregular excursions of the image that are brought about by the atmospheric instability. For photographic observation, on the contrary, it is quite necessary that the image be both sharply defined and steady for the duration of an exposure. Furthermore, a visual observer has a great advantage in being able to ignore the times when the image is distorted and to remember the moments when it is excellent, while the photographic plate indiscriminately records all the accumulated fluctuations.

"Yet in spite of the marvelous ability of the eye to catch, and the brain to retain, fleeting glimpses of extraordinarily fine detail, we cannot regard the situation as anything but unsatisfactory. The eye is not a quantitative measuring instrument, and the brain is not always objective in what it records. The accuracy, objectivity and permanence of the photographic record are as much to be desired here as in other fields of science.

"A number of possibilities exist for removing or relaxing the limitations that the turbulent atmosphere imposes on stellar photography. The most obvious of these is to try to exploit those very rare nights when a large telescope actually will perform better than a small one. Unfortunately this requires more than a steady atmosphere; it also requires that a suitable object be available in a favorable location to photograph. In the case of the moon, this immediately reduces the likelihood of such a coincidence by at least a factor of four, and; in the case of Mars, by a factor of at least 40, not allowing for the fact that the most favorable oppositions of Mars occur when it is low in the sky for the majority of the large telescopes in the world. It would be the sheerest accident: if any ordinary photograph of Mars taken with the 200-inch telescope within the next century were to show detail worthy of its tremendous resolving power!

"In contrast with the performance of the 200-inch, the chances of good seeing improve so rapidly with diminishing aperture that a telescope of 40 or 50 inches might be used effectively for planetary photography, provided photographs were taken almost continuously during every reasonably steady night. But this would require reserving a large portion of the observing time for such use. Such a program probably could not be justified except possibly for a limited time, such as during a very favorable opposition of Mars.

"Clearly the economic justification of the big telescope does not lie in its ability to resolve minute details of bright, relatively close objects. Rather, its immense light-gathering power is largely exploited for photographing objects too faint or remote in space for smaller instruments.

"We cannot hope to make the much-desired photograph of Mars by the mere expedient of building ever more powerful telescopes. How, then, may we approach the job?

"Although no ideal solution is known at present, several possibilities have been suggested and some have been tried with promising results. One is to remove directly the main cause of the problem: the atmosphere. This could be done by taking the telescope away from its traditional bedrock foundation and lifting it above most of the atmosphere in a rocket, a balloon or a high-altitude jet aircraft. Such a project has been seriously considered, but to my knowledge is not now in active progress. Many difficult problems would have to be solved, among them the weight and bulk of a large telescope and the need for a steady yet sensitive means of aiming it.

"A different line of attack, which shows considerable promise, involves the use of electronic image intensification [see "Electronic Photography of Stars, by William A. Baum; SCIENTIFIC AMERICAN, March]. The aim here is to reduce the required exposure time so drastically that the image from a relatively small telescope could be utilized. A system based on this principle has been tested at the Lowell Observatory at Flagstaff, Ariz. It yields enough intensification to permit a 30-fold reduction in exposure time. The image thus has less time for wandering about on the film, and smearing is reduced accordingly. This is essentially a closed-circuit television system utilizing an image-orthicon pickup tube connected through an amplifier to a picture tube. The picture tube is then photographed with a camera whose shutter is suitably synchronized with the picture. With this method the possibility also exists of detecting electronically the moments when the image is sharpest and building up a complete exposure out of many selected shorter intervals. It is too early to evaluate the capabilities of the new electronic methods, but doubtless much will be heard of them in the future.

"During the past few years I have experimented with a third approach in which an electronic guiding system is used to cancel out most of the motion of the image on the film. I observed that during good seeing the image of a planet tends to move as a whole, rather than to change in size or shape. This motion is erratic, but the image remains within one or two seconds of arc of some average position. Most important, the image moves slowly enough so that the design of an electromechanical servo system capable of following it appeared practicable.

"After the usual number of false starts, I assembled a guiding device and tested it on an artificial planet in the form of an illuminated hole two millimeters in diameter in a metal sheet. This spot of light could be moved in a pattern that simulated the image movement of a planet under average seeing conditions. The assembly was then coupled to a modified 16-millimeter motion picture camera and mounted on the tube of the 60-inch reflector on Mount Wilson [see drawing in Figure 1]. The planetary photographs accompanying this article were selected from the resulting exposures. The pictures show at least as much detail as was visible to the eye at about 750 power, with the exception that Saturn's crape ring was underexposed photographically.

"The device operates in this way. A small enlarging lens of about f/4.5 focal ratio is mounted on a doubly pivoted carriage [Figure 4]. The carriage permits the lens about half a millimeter of transverse motion in any direction. The two components of this motion are governed by two small electromagnets whose pulls are balanced against springs [Figure 5]. The light from the telescope forms an image in the normal focal plane of the telescope, proceeds past this plane through the enlarging lens, reflects from a partly reflecting diagonal mirror, and comes to a new focus at the film plane of the motion picture camera. Part of the light proceeds through the partly reflecting diagonal mirror and comes to a focus on a reticle, where it can be viewed by an eyepiece. Two small reflecting prisms with sharp edges project slightly into this latter beam from two directions at the focal plane and throw a small amount of the light into each one of two photomultipliers [Figure 6]. The signals from these tubes are amplified in separate direct-current channels and are fed into the electromagnet coils that determine the position of the magnifying lens. The system seeks a stable condition wherein a certain amount of light is entering each photo tube [Figure 7]. If the telescope image moves by a small amount, the amount of light entering the photocells changes, and the system responds in such a way as to cancel out this motion. This negative feedback is, of course, not capable of completely canceling the erratic motion, but it reduces it by a factor of about 10. In this way seeing fluctuations, mechanical vibration and driving errors are essentially canceled out through a frequency range extending from zero vibrations per second up to approximately two vibrations per second.

"An additional feature that is a great convenience, but not a necessity, is that there are relay contacts on the lens carriage which act as limit switches to prevent the electromagnets from having to work outside their designed range [Figure 8]. If this preset range is exceeded, the corresponding slow-motion drive of the telescope is automatically applied so as to bring the electromagnet back into the center of its operating range. Thus, once adjusted, the guider will track and center a planet image for the duration of an entire observing night. Indeed, except for focusing the image on the reticle and rotating the telescope dome now and then, the entire operation is automatic, including the timing of each exposure and the advancing of the film. The timing system is shown in Figure 9.

"Through the use of this device one of the two serious disadvantages of a long exposure is essentially removed: the relative motion of the image as a whole with respect to the film is neutralized. But it is still necessary to match the diameter of the telescope objective to the 'seeing' cell-size, so that the image will be sharply defined over the greater part of the exposure time.

"I used 16-mm. Kodachrome film with exposure times that varied from two seconds for Jupiter to 16 seconds for Saturn. Exposures were usually made at the rate of two frames per minute over a period of a few hours, and the best of the resulting images were later selected for enlargement."

Bibliography

AMATEUR TELESCOPE MAKING. Edited by Albert G. Ingalls. Scientific American, Inc., 1952.

AMATEUR TELESCOPE MAKING-ADVANCED. Edited by Albert G. Ingalls. Scientific American; Inc., 1952.

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

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