THE NEXT time you visit the station of an advanced radio ham take a look around his basement and back yard. The chances are better than even that you will find a well-made telescope. Many hams have two avocations, astronomy and electronics. The way things have been going in recent years, the two may soon merge into a single hobby.

Apollo Taleporos, whose eclipse pictures of the moon appeared in this department last September, explains: "Astronomy has borrowed so many of radio's techniques, and the aims of electronics have become so closely identified with those of astrophysical research, that you have to stop and think today before you can decide in which field you are working. I enjoy physics, particularly the electromagnetic part of it. So I do a lot of playing around in both electronics and optics. The only thing that keeps me out of the ultra-ultrashort waves is the cost of cyclotrons and sounding balloons."

Until 1946, according to Taleporos, most radio hams stuck closely to the communication side of radio. Back in 1912, when a government official was asked what part of the radio spectrum he would assign to the amateurs, he replied: "We'll stick 'em on 200 meters and below; they'll never get out of their back yards with that." As history records, in little more than two decades the hams made a back yard of the whole world, one now largely occupied by the multibillion-dollar communications industry. But by the end of World War II amateur radio as such held few remaining challenges and the hams were ripe for something new.

It came suddenly, not with a bang but a squeal. Shortly after the Federal Communications Commission lifted the wartime ban on ham activity, some of the amateurs began to hear eerie squeals on the 14-megacycle band. The squeals were heard only at night, after the band had closed down for long-distance communication. An amateur would be idly scanning a portion of his dial between two active local stations in the hope of making one last distance contact before turning in for the night when suddenly a strange "woweeee" would almost shock him out of his chair. It was unlike anything heard on the longer waves. Some evenings it would come a dozen or more times at the same point on the dial. On others it would not be heard at all.

Word passed around quickly and before long almost everyone with a supersensitive receiver equipped for tuning in continuous wave telegraph signals was listening to the mysterious wails. Then someone discovered that they came from meteors.

Meteors leave in their wake ionization trails-mile-thick cylinders of charged gas which reflect 14-megacycle waves efficiently. During the brief interval that a meteor's trail persists, a favorably located receiver can pick up the carrier wave of a transmitter which would otherwise be beyond range, for at night the ionosphere loses some of its charge and becomes less efficient in reflecting signals. Thus in effect the hams were hearing shooting stars. The discovery opened the era of radio astronomy for them.

The flood of surplus optical and electronic gear that was put on the market after the war was a big help. Much of it was sold "as is" in the form of complete units, and amateurs often had to buy a large piece of apparatus in order to get the particular meter, lens or gear train they wanted. So the whole thing was carted home and the useless parts stored away against the day when they might come in handy. Some of this "junk" consisted of parabolic antennas, photomultipliers, recording meters, prisms, filters and other odds and ends which were destined-with the help of sweat and imagination-to become priceless possessions.

Today many hams are seen more often than they are heard. Amateurs are busy tuning in celestial objects on radio telescopes, bouncing waves off the moon, tracking meteors and recording their paths automatically, detecting and measuring disruptions on the sun's surface, plotting the orbits of eclipsing binaries and measuring the apparent magnitudes of variable stars with a speed and precision which would have startled professionals in the days when Harlow Shapley suggested that the cepheid variable might make a good yardstick.

Just as meteor observing attracted the radio hams to astronomy, so the photoelectric cell made electronic addicts of many optical men. For more than four decades members of the American Association of Variable Star Observers have been looking at these stars through their small telescopes. They have made more than a million and a half observations of some 600 variable stars. But the job takes good eyesight. Now electronics can do it much better. With the variety of sensing tubes now on hand, electronic gadgets can see farther, faster and more sharply and keep at it longer than the human eye. Moreover, electronic eyes can "see" in invisible parts of the electromagnetic spectrum and can tolerate blinding intensities.

One of the first amateur astronomers to go all out for electronics was John Ruiz of Dannemora, N. Y. After several years of mixing vacuum tubes and lenses, he recently said, with only slight exaggeration: "Whatever you can do mechanically you can do better and cheaper electronically." Whereas in mechanical amplification accuracy to one part in 100,000 is considered very good, in electronic amplification accuracy to one part in 100 million is routine. It takes high craftsmanship to make a telescope objective that yields a magnification of 300 diameters with good resolution, but a beginner can build on his first try an electronic gadget capable of amplifying a million million times with equally good "resolution."

Electronics has also taken a load off the amateur astronomer's pocketbook. For example, $25 will equip an amateur to pick up from the National Bureau of Standards' Station WWV time signals accurate to two parts in 100 million. How much would a pendulum clock of comparable accuracy cost, assuming, of course, that the market afforded one? The effect of the merger between astronomy and electronics has been to bring the amateur of modest means to a more nearly equal footing with his professional colleague in equipment.

A photoelectric photometer which Ruiz built for taking the guesswork out of variable star observing illustrates how simply optics and electronics can be combined to make an instrument of extraordinary power. As Ruiz says, you can hitch a photometer of your own make to a star.

"Of course, the star that you select should be one showing some action, i.e., a variable star. Some variable stars go through a complete cycle in the course o f a few hours (e.g., the Beta Canis Majoris type), and you can make a full set of measurements for a complete 'light curve' in one night, weather permitting. Other stars of the eclipsing type have periods of a few days or longer. These are more difficult to follow and may require many nights of observing for a complete light curve. In the northeastern U. S. a series of observations on a single star may span two years or more, because nights ideal for photometry are rare.

"As an example of what an amateur can do with relatively modest equipment consider the light curve of No. 12 Lacertae, a short-period star of the Beta Canis Majoris type [see upper illustration at right]. This curve was taken in the course of one night. Ten curves like it were taken in the summer and fall of 1951 and they proved of value to the Dutch astronomer C. de Jager in making a new determination of the 'beat period' of the light variation. This star exhibits fluctuations both in amplitude and period, or, as radio hams would say, it exhibits both AM and FM.

"Another type of variable is illustrated by the complete light curve of Mu Herculis, an eclipsing variable with a very short period-two days and one hour. The curves were taken in two colors, blue and yellow, corresponding roughly to the respective photographic and photovisual magnitudes. These curves promise to be of value in a more accurate determination of the orbital elements of this system.

"As some astronomers have recently pointed out, visual observation of variable stars is now outmoded, at least for those of short periods and narrow range. The photoelectric method is at least 10 times as accurate and far more objective, particularly in ascertaining the brightness of stars of different color or spectral class.

"To make graphs like these, you must have, first of all, a telescope solidly mounted in the manner advocated by Russell Porter. My setup is shown in Roger Hayward's illustration [left]. The telescope should be provided with an accurate clock drive and slow motions in both declination and right ascension. A reflecting telescope is preferred to a refractor, because violet and near ultraviolet light, to which the cesium-antimony photocell is most sensitive, is reflected by the aluminum surface of a mirror more effectively than it is transmitted by a refractor. Incidentally, the prism of a Newtonian should be replaced by an aluminized flat for the same reason. For the conventional eyepiece and eye we substitute a photometer head, or light box, which consists mainly of a holder for filters and the photomultiplier tube.

"In principle the stellar photometer is little more than a glorified exposure meter of the type used in making photographs. Like omnia Gallia, it consists of three parts: the photometer head, which corresponds to the light cell of the exposure meter; a direct current amplifier to build up the faint energy received from the stars, and an indicating device, usually a milliameter, which is read by eye, if possible by the eye of an assistant.

"The construction of the photometer head calls for no special tools and is easy if the amateur can lay hands on a small junk camera. Mine was made according to suggestions from William A. Baum of the Mount Wilson and Palomar Observatories. It is built around a camera shutter provided with an iris diaphragm [see inset, Fig. 2]. Note the use of the 'field lens,' which serves the double purpose of forming an enlarged image of the star (centered on the iris diaphragm) and of projecting on the photocathode an image of the fully illuminated mirror rather than a pinpoint image of the star under observation. This compensates for the granular structure of the photocathode, which is not equally sensitive at all points. You may compute the focal length of this field lens to give an image of your object glass or mirror about five millimeters in diameter, using nothing more than the elementary optics you learned in high school. Note that the gate holding the multiplier can be swung to one side when centering your star on the diaphragm. It is provided with an automatic shutter to cut off all light to the tube when this is done, for if strong light strikes the energized photocathode, it may become temporarily unstable o even permanently damaged.

"My photometer uses a nine-stag lP21 photomultiplier which works on the secondary emission principle, as explained in this department last March. Each stage amplifies the signal by a factor of four or five, so that the over-al gain in signal strength is enormous.

"The energy we receive from a faint star is so small that even after immense amplification by the dynode stages of the lP21 tube it may amount to no more than amperes. Minute currents of this order of magnitude can be measured by ultrasensitive galvanometers and electrometers. But these instruments are costly, require rigid mounting and are delicate. The output of the photomultiplier tube is usually amplified.

"Two properly chosen vacuum tubes in a well-designed circuit will provide amplification of a million and permit the use of a meter which is driven to full-scale deflection by one milliampere. Many of these rugged little meters are on the market and can be purchased for less than $5.

"Gerald E. Kron of the Lick Observatory has designed a two-tube, direct-coupled amplifier specifically for photometric use which is about as simple and foolproof as the instrument can be made. The complete circuit, together with instructions for building it, a list of the parts and full operating information, comprises a chapter in Amateur Telescope Making, Book Three.

"With these three units completed, all that remains is to hook them together, substitute the photometer head for the eyepiece, select a star and have fun. If you have both a persuasive personality and a kindly disposed wife, she can take readings while you do the guiding with the auxiliary telescope. Of course you may choose to buy a Brown potentiometer for about $600 as an alternative- but I could not induce the director of the budget in my household to approve this expense. So most of the time I do all the work myself. As is evident, stellar photometry is an ideal project for two amateurs, one a gadgeteer to build the apparatus and the other a researcher -who enjoys the role of chief observer.

"To meet the present high standards of professional observation, an-amateur must apply to his readings certain corrections which were of no consequence in the era of visual work. These have to do with atmospheric extinction, reduction of time to the sun and non-linearity of the meter and amplifier. But a serious worker will find many a helping hand among his kind both in amateur and professional circles. The dividing line between a professional and an amateur is very dim indeed, for only an 'amateur' would ever become a professional. James Stokley of the General Electric Company once said that the difference between an amateur astronomer and a professional is that the amateur is sorry when it is cloudy."

OBSTRUCTIONS in telescope tubes, such as diagonal and secondary mirrors and their mechanical supports, not only cut off light but also cause injurious diffraction. Horace E. Dall has pointed out in Amateur Telescope Making-Advanced that diffraction vitiates the image and reduces contrast of fine detail, such as lunar and planetary features. The radial spikes that project from images of bright stars on some photographs are evidence of diffraction caused by telescopic obstruction. In general, reflecting telescopes, most of which have central obstructions, give images with lower contrast and slightly poorer definition than those of high-quality refractors.

The late planetary astronomer William H. Pickering described a diffraction experiment which emphasized the impairment of seeing by obstructions. First he deliberately increased the diffraction effect by enlarging the obstruction area of the diagonal mirror with a paper mask extending beyond the mirror. Then he entirely eliminated the obstruction of the diagonal mirror and its supports by another device-a mask which let the incoming rays of light reach the mirror only through a relatively small off-center opening. In spite of the reduction in the amount of light that reached the mirror, the seeing was greatly improved. Adopting this working principle, A. E. Douglass, another planetary astronomer, masked the aperture of a 86-inch telescope in such a way as to leave uncovered only an off-axis circle 18 inches in diameter [see drawing at upper left in Fig. 3]. The result was a net gain in visibility of fine detail despite the loss of brightness.

Astronomers have noted these evil effects of central obstruction for many years, but it is not easy to build large off-axis telescopes to avoid them. In recent years advanced amateur observers of the planets and moon who have exhausted the fullest powers of conventional obstructed telescopes have been inventing new telescope designs for reducing diffraction effects. One of these is J. S. Hindle of England, son of the late amateur astronomer J. H. Hindle He writes:

"When conventional types of reflecting telescopes are used visually on planets, especially with large apertures, the finest detail is usually best seen by using an off-center diaphragm to cover the secondary mirror and its supports, as described by Pickering. However, the arrangement is far from perfect, as the large block of glass in the primary mirror is rarely in a state of thermal equilibrium and definition consequently suffers. An alternative for obtaining an off-axis section of a paraboloid is to coat a spherical mirror with an uneven layer of aluminum. Excellent in theory, this is open to the objection that the thickness of the aluminum deposit must be extremely accurate to get optimum results. The Herschelian reflector, used with a meniscus correcting lens in the return beam [see drawing at upper right, Fig. 3], seems to offer an answer. Moving the lens to or from the mirror gives under or overcorrection for spherical aberration. Although the scheme gives remarkably good results with moderate apertures, it proves cumbersome with large instruments, the observer perforce being perched high on a ladder or platform in the dark. The same dangerous and uncomfortable situation arises when an off-axis paraboloid is used.

"The skew Cassegrain was first used to avoid central obstruction by Karl Fritsch of Austria, who made many small ones about six inches in diameter, and A. A. Common of England, who made a 12-inch from which he claimed to get good visual results. These instruments used two spherical mirrors [see cross section second from top, Fig. 3]. Although the aberrations of the secondary tend to cancel the spherical aberration of the primary, the method fails when tried on large apertures, the residual spherical aberration being such that critical detail is unobtainable under high magnifications. Tilting the secondary tends to minimize spherical aberration but introduces astigmatism, which causes star images to appear as short straight lines.

"Another modification which at one time looked promising was an elliptical secondary about two thirds the diameter of the primary [see cross section third from top]. This large secondary was necessary because only an off-center section of it was used. The scheme failed on large sizes, due to flexure of the secondary, which could be supported only at the edge. The elliptical secondary also is very difficult to test and figure.

"To dispose of these defects of the off-axis telescope I have designed and constructed a Cassegrain-Schmidt combination [see cross section fourth from top]. It has two spherical mirrors and a small correcting plate similar to a Schmidt plate to cure spherical aberration. In sizes larger than about eight inches this type will give far sharper detail and stand far higher magnification than the conventional Newtonian and Cassegrain types. My 12-inch will stand 600 diameters on most nights and still give an image nearly as sharp and crisp as when using only 60 diameters. Tested against three different 12-inch reflectors and one 15-inch, it has easily outclassed the performance of them all when planetary detail was being observed. The data for a 12-inch telescope of this type are as follows: Focus of primary, 10 feet or more. Diameter of convex, 8 inches. Radius of curvature for convex, 62 inches. Cone cut off by convex, about 3' focus. Diameter of correcting plate, 8 inches; thickness, about 1/4 inch.

"If these proportions are approximated, the primary mirror may be used for testing the figure of the correcting plate. The angle of the return cone of rays, when an illuminated pinhole is placed at the center of curvature of the primary, will be identical with the angle of the cone from the secondary when the finished instrument is used at infinity. The amount of deviation from flatness that must be imparted to the correcting plate can be calculated from the familiar formula , with one important difference. The r in this case does not refer to the semi-diameter of the primary; it refers to the distance from the optical axis of the system to the farthest point on the circumference of the primary.

"When testing the correcting plate the spherical primary is set up as usual for the Foucault test but using about six inches' separation between the pinhole and knife-edge [bottom drawing]. The plate is then placed between the primary and knife-edge, at such a distance that it just fills with light when viewed from the center of curvature, without any of the mirror being visible outside the plate. In the example given the plate must be figured until the primary assumes the appearance of a hyperboloid, with the outer rays coming to a focus about 7/8 inch longer than the central rays. It will be necessary to attack both sides of the plate to obtain this deviation, if the plate is made from polished plate glass. It is advisable to cut several square pieces and test for astigmatism in the return cone of rays from the primary at center of curvature. The best shape of polisher for the plate is shown in the illustration. If the plate is mounted on a rotating spindle, it may be quickly roughed to shape with the ball of the thumb, and afterward finished off with the polishing pad.

"If the plate shows traces of astigmatism when finished, do not scrap it but rotate it in different positions when in the telescope, as only half of the plate will be in use and a good diameter usually can be found.

"The best test for the convex is King's, but a fair idea of the figure may be obtained by placing the pinhole at four times the focal length of the primary away from the primary and intercepting the return cone of rays with the convex, so as to reflect the cone back to the edge of the primary. On passing a knife-edge across the apex of the cone, an even darkening of the light should be observed. The big snag to this method, of course, is that a long testing room is required. The primary should be aluminized for the test, because there are two reflections.

"A slightly positive meniscus lens may be substituted for the correcting plate, but it will give a rather curved field of view. The chromatic aberration caused by the correcting plate is so small as to escape detection with quite high powers.

"The stops shown between the eyepiece and the plate are very important, as objectionable sky flooding will occur if they are missing or imperfectly positioned.

"Moving the correcting plate nearer the convex increases correction for spherical aberration, and moving it away has the reverse effect, hence good definition will not be attained until this adjustment has been made perfect.

"To collimate the instrument it is necessary to place a button or small circular object exactly in the center of the primary, at the open end of the tube. Both mirrors are then adjusted until the reflection of the mirror and the button are seen concentric in the convex, looking from the eyepiece drawtube with the eyepiece removed."

The off-axis Cassegrain shown in the second drawing from the top was invented in 1876 by I. Forster and Karl Fritsch of Vienna and was known as the brachyte. In the December 1, 1952, issue of The Strolling Astronomer, organ of the Association of Lunar and Planetary Observers, Guenter Roth and E. L. Pfannenschmidt described a modern German variation called the neobrachyte. Roth's f/20 "neo-bra" has an eight-inch spherically concave f/12 primary mirror, its axis tilted to an angle of 3 degrees, 16 minutes and 30 seconds of arc with the incoming light. The four-inch f/24

spherically convex secondary mirror has the same radius of curvature as the primary and is tilted to an angle of 12 degrees, 56 minutes from the new or deflected axis of the primary (though the three axes involved are not in the same plane). The secondary is separated from the primary by a distance of seven times the diameter of the primary. Astigmatism is eliminated by deforming the secondary to a slight cylindricality with a screw pressing against its back along one diameter. This subterfuge, which avoids the necessity of figuring an aspherical correcting plate, is the secret of the neobra.

According to the authors an eight-inch neo-bra will show perfectly round, concentric star images at powers of 300 to 450. Larger sizes require a shorter focal ratio, ray tracing and a Dall-Kirkham primary. Excellent star images were given by a 12-inch neo-bra at a power of 500.

Roger Hayward, who made the illustration from Hindle's sketches, comments: "I recall J. A. Anderson (who had charge of the optical parts of the 200-inch Hale telescope) saying that the ideal size of telescope for visual observation was around 10 inches. In a smaller telescope the image suffers from too little resolving power, in a larger one the seeing deteriorates. This is because atmospheric waves that mess up the air commonly are of such dimensions as to make the image from a 10-inch disk wobble about at a rate which the eye can follow. At twice this size or larger the image will go in and out of focus as the light from two sides of the disk is refracted together or apart. I feel that this, more than the absence of obstruction, is likely to have been one reason why the 13-inch diaphragm opening improved the seeing with the 36-inch telescope."

Anderson has described a simple method for observing atmospheric waves that impair good seeing. These waves are believed to be ripples at the interfaces between moving layers of warm and cold air high aloft. Anderson points a small low-powered telescope-for example with one-inch aperture, eight-inch focal length and a low-powered eyepiece-at a bright star, holding the eye end with one hand, the objective end with the other. Then he swings the objective end in a circle about a quarter inch in diameter at the rate of four or five revolutions a second. Due to the persistence of vision, the star image is drawn into a nearly complete circle if the rate of rotation is correct. You soon learn to judge the state of the seeing by the number of bright patches in the circle. The bright patches correspond to the moments when the concave crest of the wave is in line with the telescope so that it refracts the sides of the interrupted disk together, and the darker portions correspond to the convex trough of the waves, which Anderson assumes from experience are about six inches in length. In good seeing the frequency of the bright patches with this scintillometer ranges from a few to 25 or 30 a second, while in bad seeing there are often 150 a second.

Much has been written about the evil effects of central obstructions on visual observation, but there is no agreement about the effects. Laboratory experiments which exclude the effects of the turbulent atmosphere do not agree closely with tests at the eyepiece out-of-doors. The laboratory tests seem to prove that there is no serious impairment of image quality until 20 to 30 per cent of the diameter of the aperture is obstructed. On the other hand, E. K. White of British Columbia found at the eyepiece that even the 17 to 25 per cent obstruction in the typical Newtonian telescope caused pronounced diffraction effects on the images of planets. With a reduced diagonal which obstructed but 10 per cent of the aperture diameter, he found that the image quality was greatly improved. The editor of The Strolling Astronomer, in which the experiment was described, remarked that White had observed little-known delicate features on Saturn about as well with a nine-inch telescope as he had been able to do with an 18-inch telescope.

Does the factor described by Hayward-the length of the atmospheric waves high aloft-have anything to do with this equality? David W. Rosebrugh, a veteran observer, believes that it does. He thinks that the maximum useful aperture for visual use is six inches, because larger telescopes are more sensitive to the atmospheric waves.

After reading a score of articles on the subject, as I have just done, you may come to feel that only two facts have been isolated: (1) unobstructed telescopes are better than obstructed ones; (2) the effects of obstruction are confused by the effects of size of aperture.

Bibliography

ELECTRONICS: EXPERIMENTAL TECHNIQUES. William C. Elmore and Matthew Sands. McGraw-Hill Book Company, Inc., 1949.

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.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.