IN RECENT YEARS MANY BACKYARD observatories have acquired a new aspect. The circle of cane-bottomed chairs which formerly welcomed the visitor has all but vanished in favor of the relay rack, high-gain amplifier and data recorder. Even the place where the visitor once put his eye has been usurped by a small black box. The telescope continues to sweep the sky from its central spot beneath the dome, but the whir and click of automatic devices now accompany the stars on their silent march.

Today it is not unusual to find amateur astronomers operating a dozen or more electronic instruments which were all but unknown before World War II. The trend was launched when electric motors developing a fraction of a horsepower began to displace mechanical clocks for driving small telescopes in right ascension. Then came vacuum tubes, tied together by feedback circuits, to regulate the motors. Amateurs next discovered that the photoelectric cell makes a good substitute for the eye, particularly on fatiguing jobs such as keeping a star centered on crosshairs. The introduction of electronics into amateur astronomy suggested a logical new development: amateur radio astronomy.

Many amateurs have expressed interest in building a radio telescope, but few have actually built one. The chief barrier appears to be the lack of a simple design. If any amateur has built such an apparatus with the facilities of the average basement workshop, this department would like to hear about it. Meantime we have learned of a group in Manhattan, Kan., which has completed a related project: a device to count meteors electronically. The apparatus is described by W. S. Houston, editor of The Great Plains Observer, official bulletin of amateur astronomers in the Midwest.

"When a meteor flames through the upper atmosphere at a height of 50 to 100 miles," writes Houston, "it leaves an ionized trail in the air which may vary in diameter from a few feet to a mile or more, depending on the size of the meteor. The mechanism responsible for the trail is not fully understood. Some trails last several minutes. Others vanish after a second or less. In any case, such trails act as excellent reflectors for radio waves, especially those in the range of 80 to 90 megacycles. Short-wave transmitting stations too distant for normal reception come in loud and clear when their signals are reflected by meteor trails. The duration of the propagation depends on how long the displaced electrons bounce around before recombining with atoms of gas.

"There is nothing new about this, of course. Radio 'hams,' at least the handful who go in for experiments aimed at extending the bounds of knowledge, have long been accustomed to hearing weak signals from long distances suddenly build up to window-rattling proportions and to developing psychological resistance to the brief yowls which occasionally punctuate otherwise clean transmissions. As long ago as 1940 the hams correctly ascribed these weird effects to meteors. Cliff Simpson (WOYUQ), a pioneer in the ultrahigh-frequency portion of the radio spectrum at Manhattan, Kan., had been seriously playing with meteor propagation for several years when the great Giacobinid meteor shower came along on October 9, 1946. The fall created something of a sensation in ham circles. Radiotelephone signals came through from stations hundreds of miles away on frequencies normally limited to line-of-sight transmission. The reflected paths were so solid that many lengthy conversations took place.

"As is often the case, particularly in radio, the scientific world trailed the amateurs. Meteor propagation failed to attract serious interest until the National Bureau of Standards a few years later installed a 20-kilowatt station at Cedar Rapids, Iowa, for the study of forward-scatter propagation between the Midwest and Washington, D.C. Numerous brief intervals of high transmission were immediately observed; thus the Bureau independently discovered what the hams had known for years.

"Last fall the local section of the American Meteor Society at Manhattan decided to take advantage of the phenomenon for making a systematic count of meteors. Thanks to the invaluable cooperation of two local hams, Simpson and Ben Mullinix, we have installed an automatic system not only for making a continuous, round-the-clock count of meteors, but also for monitoring auroral effects.

"On a clear night, anywhere on earth, you may expect to see about six meteors per hour. As many as one per second may appear on some nights. Because the observer's field of view is limited, meteors counted at any one location represent only a minute fraction of the total; fall. Rough calculations indicate that some four million meteors bright enough to be seen cross the sky every 24 hours.' By including those brighter than the 10th magnitude the estimate rises to four billion. Counting a reliable sample of that many invisible objects within the range of a single observer was impossible before the development of very sensitive photographic plates, and it was not easy thereafter. Background light from the sky limits the time during which sensitive plates can be exposed before they become fogged and thus also limits the number of meteors which can be detected with a single plate. The radio counter avoids these limitations, although it introduces a few of its own.

"The transmitter we use is the Bureau of Standards station at Cedar Rapids. The frequency is 49.8 megacycles. The station is beamed toward Washington, D.C., by a rhombic antenna, a diamond-shaped array of wire supported by telephone poles which creates a highly directional pattern of radio waves. However, enough energy leaks from the sides of the structure to send a strong signal in our direction. Actually, because of the earth's curvature, the signal crosses Kansas at a height of about 15 miles, so that we cannot receive it directly. But meteor trails reflect part of the signal to our station.

"We pick up reflected signals by a directional antenna of the Yagi type, about the size and same design as a conventional television antenna. The antenna is made of aluminum tubing, one tube being cut in the middle for the receiving dipole, a slightly longer one backing the dipole as the reflector, and two progressively shorter ones mounted in front as directors. The geometry of the assembly is adjusted to the frequency of the transmitter for maximum response. The Yagi is mounted on a steel support for rotation in both azimuth and elevation. The support consists of a pair of telescoping steel pipes, one of which is sunk in concrete. The array is turned by hand. The antenna stands about 10 feet above the ground. Normally we point it toward the horizon, but it can be elevated to any angle desired. A 50-ohm coaxial cable carries the signal into the observatory.

"The incoming signal is fed to a shortwave converter tuned to 49.8 megacycles and locked on frequency by a quartz-crystal oscillator. We normally detect a signal on this frequency only when the signal is reflected by a meteor trail. Trying to tune a receiver by occasional signal bursts proved too much for our patience, so we decided that crystal control is a must. Parts for the converter cost $12.50. The receiver is an Army surplus job modified considerably by Simpson. It is also crystal-controlled, and it is fed by the converter at seven megacycles. The receiver features a so-called 'squelch' circuit which in principle resembles the conventional automatic volume-control. Instead of maintaining the output at a uniform level, the squelch circuit cuts the sound completely in the absence of a signal and turns the set on fully when signals appear which are higher than the average background noise. A control is provided for adjusting the squelch to any desired signal level. Hence the output of the receiver is dead until a signal appears. Finally the receiver is equipped with a variable oscillator, the output of which can be mixed with the signal. The pitch of the resulting beat note can thus be adjusted as desired.

"Audio output from the receiver is fed to a control circuit which in turn actuates a sensitive relay. The control circuit is built around a 6SQ7 tube, as shown in the accompanying illustration [next page]. The arrangement functions both as a detector for converting the audio signal to direct current and as an amplifier for energizing the relay. A delay element is built into the control circuit which discriminates against groups of signal pulses which persist for less than three seconds. Such groups register a single count. The delay arrangement consists of a one-megohm resistor bridged by a two-microfarad capacitor connected between the Stancor choke and ground. If control of the response time is desired, the fixed resistor may be replaced by a rheostat. The relay is of the type used for controlling model airplanes by radio. They are sold by most hobby shops. The transformer [T₁ in the illustration] is a conventional output-transformer hooked up backward.

"The counter is a standard telephone job of the rachet type that is available on the surplus market. It is energized through the relay of the control circuit and reads to 9999. A loudspeaker is bridged across the output of the receiver so that the observer can monitor meteors by ear.

"The system is not free of error. Lightning strokes, even at appreciable distances, are counted along with meteors The difference can be spotted by ear. Meteors cause a beat note; lightning makes a short, crackling hiss. The level of the squelch setting has an important effect on the count, and it tends to drift. As an aid in maintaining the optimum adjustment we inserted a milliammeter in the circuit. The operation of light switches, plugging or unplugging appliances into the power line, and so on are also counted. When the sporadic E layer of the ionosphere appears, the relay pulls down, putting us out of business. In short, any noise greater than the thresh old at which the squelch circuit is set to operate will register as a count or, if it lasts long enough, mask a count.

"A number of alternative methods are available for reducing the error, if one is willing to dig into his wallet. For example, Simpson has suggested a system based on two receivers. The outputs of the two units would be connected so that the signal voltages would be 180 degrees out of phase and would buck one another. One receiver would be locked to the frequency of the transmitter by a quartz-crystal oscillator. The other would be tuned to a slightly different frequency. Both receivers would pick up equal amounts of noise. The resulting noise signals would oppose each other in the control circuit and disappear. In contrast, the desired signal would appear at the output of only one receiver (the one tuned to the transmitted frequency)-and hence would pass through the control circuit unopposed. A pair of BC 454 surplus receivers should work nicely in such an application. Only one would require modification for crystal control; the other would function solely as a noise detector.

"We found that many meteors, particular]y those of long duration, produced a warbling note. At first each fluctuation was counted as an individual trail. This was cured by the delay circuit, which holds the relay closed during momentary lulls and prevents overcounting. Too much delay can be introduced. We compromised on a delay of three seconds. If two or more meteors fall within this interval, our equipment counts them as one.

"When the system first went into operation we asked: 'Are we really counting meteors?' We got the answer by watching near the horizon in the direction of Cedar Rapids. Within a minute or so a faint streak of light appeared in the sky. The counter clicked and the speaker emitted a loud beep. The system worked! In general we observed that any meteor which can be seen makes an abnormally loud beep. We estimate that the system is capable of counting meteors down to the ninth magnitude. We have not explored the angular range over which we can get radio-visual coincidences, but one night we observed a coincidence at an altitude of 45 degrees. Incidentally, visual meteors more than 30 degrees off the axis of the Yagi are not counted. None are heard if the antenna is turned at right angles to the direction of the transmitting station.

"The system operates over an area considerably greater than an observer can cover visually. Visual observations are limited to a cone some 30 degrees in diameter. The beam of our system covers about 40 degrees. When the radio cone is projected to the area containing the trails, it is obvious that far more sky is under surveillance than a visual observer beneath the area could command. The counting rate of the system is accordingly higher, even for meteors of the first magnitude, than would be reported by a visual observer. If the squelch circuit is turned off, the normal count rises to as much as 200 per hour, and can be heard by listening for trails under noise With the squelch fully on, only brighter meteors are counted or heard; they run from 25 to 500 per day.

"The system can also be used for detecting auroras. When the northern lights are active, even if not visible from Manhattan, we can turn the Yagi north and count meteors Signals directed to the north by the transmitter apparently bounce off meteor trails and the aurora reflects the signals back to us. We accordingly monitor auroras by pointing the Yagi to the north and keeping it there as long as the counter operates.

"The apparatus has now been in intermittent operation for about 10 months, with gaps in the records representing thunderstorms and time out for adjustments, modifications and repairs During the early passes of Sputnik I and Sputnik II we shut the system down so that we could borrow the Yagi for satellite tracking. The station is now fully equipped for satellite work, thus removing this handicap to the meteor program.

"Some of the raw data gathered during the last four months of 1957 are shown in the accompanying chart [Bottom of Figure 4]. It is too early to estimate how good these records may be. We are still looking for sources of error and attempting to learn the best methods of operation. Our goal is a full year's run. Amateurs who would like to hear meteors on ordinary receivers should tune to the 20-megacycle channel of WWV or to a comparable channel of some other high-powered transmitter which has either faded out or almost out. Bursts of strong signal will mark the fall of meteors. Some patience may be needed, particularly if one is uncertain of the precise point at which to tune the receiver. Those who detect the Cedar Rapids beam all the time of course cannot use it for meteor counting."

In making a reflecting telescope the amateur first grinds one disk of thick glass on top of another with successively finer grades of abrasive between. The grinding action is such that the upper disk of the pair develops a slightly concave spherical surface. This surface is polished on a lap of filtered pitch charged with rouge and then, by continued polishing, deepened toward the center just enough to transform the sphere into the desired paraboloid.

In most small telescopes the difference between the sphere and paraboloid amounts to only a few millionths of an inch. For measuring such fine dimensions the amateur uses a remarkably primitive instrument first suggested by the French physicist Leon Foucault. Basically the device consists of an artificial star and the edge of a knife. Foucault's star was merely a pinhole illuminated by a candle. Rays from a pinhole situated adjacent to the center of curvature of a spherical mirror come to focus at a point directly opposite the pinhole on the other side of the mirror's optical axis. When the eye is placed behind the focus, the spherical figure appears flat. If a knife-edge is passed through the focus, the disk darkens uniformly. But any departures from the true sphere deflect the rays so that the departures appear as hills or valleys magnified about 100,000 times.

Most amateurs prefer to work with more sophisticated versions of Foucault's apparatus, although the truly patient fellow can get along with the classic setup. C. N. Fallier of Hicksville, N.Y., submits a version of the instrument.

"Like numerous other amateurs," he writes, "I have read reams of literature on grinding, polishing and figuring mirrors for reflecting telescopes. While most of these papers have been manifestly clear as to the mirror itself, the Foucault knife-edge tester is invariably dismissed as a rudimentary device-usually described as a tin can with a lighted pinhole and a movable knife-edge fitted to a hand-drawn scale.

"After many attempts (with as many failures) to make accurate measurements with such home-built testers, I decided to make the job easier for myself. My design uses a light bulb, pinhole and knife-edge, and so does not depart in essentials from Foucault's arrangement. But I have combined them in a stable mechanical assembly which is convenient to use, easy to make, and which yields highly reproducible measurements. The design provides for both longitudinal and transverse movement of the knife-edge under the control of feed screws [see illustration in Figure 5].

"The device resembles the compound rest of a lathe but is based on the principle of kinematical design described by John Strong in his Procedures in Experimental Physics. This principle states that a body must have at least six minus n points in contact with a second body if it is to have only n degrees of freedom. The screw for advancing the carriage in the direction of the mirror has been accordingly provided with five points of contact with respect to its reference body, the base. This is accomplished by beveling one arm of one of the two V-notches in which the screw rides. The bevel engages the annular V-groove of the screw shaft and accounts for two contacts. The unbeveled arm of the same V-notch makes the third contact with the screw. The flat arms of the remaining V-notch provide the remaining two for a total of five contacts. Thus the screw has one degree of freedom, that of rotation. In contrast, the carriage which rides on the screw is provided with six points of contact with respect to its reference body (the screw and the base): five with the screw and one by the ball in contact with the base proper. Thus the position of the carriage is uniquely determined by each setting of the screw. The design of the transverse carriage conforms to the same principle.

"The only machine work involved in making the unit is the removal of threads from each end of the pre-threaded screw stock (for contact with the V-notch bearings ), and the cutting of the annular V-groove in the shaft. All other work can be accomplished with a hacksaw, file and drill. The dials and knurled knobs are from surplus gear.

"The light source is a conventional incandescent bulb of the type used in sound heads of 16-millimeter movie projectors. The beam is deflected at a 90degree angle by a microscope coverglass. This is not the best possible arrangement because the two surfaces of the glass reflect a double image of the pinhole. I plan to overcome this by replacing the cover-glass with a beamsplitter of the prism type. A piece of ground glass is mounted between the lamp and pinhole to prevent a magnified image of the lamp filament from appearing at the focus."

Bibliography

AMATEUR TELESCOPE MAKING-BOOK ONE. Edited by Albert G. Ingalls. Scientific American, Inc., 1957.

THE RADIO AMATEURS HANDBOOK. American Radio Relay League, 1957.

Suppliers and Organizations

The Society for Amateur Scientists
5600 Post Road, #114-341
East Greenwich, RI 02818
Phone: 1-877-527-0382 voice/fax

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