"My interest in observational astronomy," Little writes, "began about five years ago when I acquired a 10-inch reflecting telescope. After a few months, when the thrill of 'just looking' began to pall, I fitted the telescope with a plateholder so that I could try deep-sky photography. This proved disappointing. My successes averaged about 1 percent of my tries, even when I set up the apparatus far beyond the range of city lights. Wind was my big enemy. A gentle breeze can vibrate the tube of a long telescope enough to destroy the sharpness of the photographic image. In addition I learned that big telescopes are difficult to move and to set up at distant locations.

"Such problems disappeared when I acquired my Questar, which is a miniature telescope weighing only seven pounds. It has an electric motor for tracking, or rotating the lens system in step with the apparent motion of the stars. To power the motor in the field I constructed a variable-frequency oscillator that operates from the storage battery of my car. This device enables me to guide the telescope-that is, keep it locked onto a desired star by changing the frequency of the oscillator [see "The Amateur Scientist," October, 1959]. A set of brackets was improvised for attaching the 35-mm. cameras to the tube of the telescope. I also made a set of illuminated cross hairs for the eyepiece. To make a photograph of the sky I center a selected star on the cross hairs and then adjust the frequency of the oscillator to keep the star in position as long as the shutters of the cameras are open.

"The beauty of the equipment is in its compactness, solidity and ease of transportation and use. I have photographed deep-sky objects in winds up to 20 knots without adverse effects. I am always able to work from a comfortable sitting position. As many as 20 photographs of deep-sky objects have been made during the course of a single night.

"Experience has taught me a few basic rules for those who may be inclined to try this hobby. First, exposure time must be determined experimentally. Exposures that are reported by others can be grossly misleading. Many amateurs, for example, report exposures of one hour 45 minutes at a lens opening of f/5 for the Horsehead Nebula in Orion. My photograph of this formation was made in 30 minutes at f/5 [see Figure 1]. Long exposures tend to emphasize guiding errors-that is, to allow the movement of star images with respect to the film. Excessive exposure time also permits scattered light from the sky to fog the film and degrade the contrast of the image. My best photographs have been made with Eastman Kodak Spectrographic film Type 103 aE. My second preference is Tri-X.

"Effective tracking requires that the polar axis of the telescope be parallel with the earth's axis of rotation. This adjustment can be difficult to accomplish in the field. I have hit on a method of making the adjustment that works quite well. First I set the declination axis of the telescope mounting to the latitude of the location. Then, with the lens tube set at 90 degrees declination, I center the cross hairs as accurately as possible on the pole star by adjusting the position of the whole telescope.

"The tube is now rotated in declination and right ascension, without disturbing the position of the mounting, until a bright star near the celestial equator is centered on the cross hairs. This is the part of the sky that includes the path of the sun. If the polar axis of the instrument is not accurately parallel with the earth's axis, the selected guide star will drift in declination away from the cross hairs.

"When such drift is apparent during an interval of less than 1O minutes, I shift the azimuth of the telescope base in either a clockwise or a counterclockwise direction and again observe the guide star. If the altered position has accelerated the drift, I next rotate the base in the opposite direction. When no drift is evident in 10 minutes, I make my exposure. Drift in right ascension is easily corrected by altering the speed of the telescope's motor with the variable-frequency oscillator. The azimuth adjustment must again be altered if the telescope is directed to higher or lower angles of the sky because of variation in the refraction of the atmosphere.

"I always use the highest possible magnification for observing the guide star. Actually the magnification should be increased in direct proportion to the focal length of the camera lenses. I also use a rheostat for controlling the illumination of the cross hairs, a useful provision when guiding on a faint star.

"Incidentally, few commercially available camera lenses are well corrected for blue light at full aperture. Blue rays do not focus sharply at the lens setting that is correct for yellows and reds. I minimize this effect by stopping all lenses down to at least f/5 when making exposures with black-and-white film. This is not always necessary in the case of color film, which appears to have better antihalation qualities. Some good color shots have been made with openings as large as f/2.8."

While Robert Little was learning how to make deep-sky photographs the editor of this department was spending most of his free time experimenting with the helium-neon laser previously described here [September, 1964]. Lasers of this type are in effect electromagnetic oscillators. Essentially they consist of an amplifier tube and a resonant cavity. They emit continuous beams of intense coherent light at 6,328 angstrom units and open new experimental opportunities for amateurs in several disciplines. My own interest in recent months has centered on improving the performance of the apparatus- particularly by increasing the useful life of the amplifier tube-without substantially increasing the cost over that of the unit I had originally constructed.

The amplifying portion of my apparatus consists of a straight gas-discharge tube, equipped with cold electrodes, that contains a mixture of helium and neon in the ratio of seven to one at a pressure of 1.8 to 2.7 torr, depending on the diameter of the tube. The ends of the tube are closed by plane windows set at an angle equal to the trigonometric cotangent of the index of refraction of the window material, which may consist of any clear pure glass or of fused quartz. The windows are attached to the tube by epoxy cement. The resonant cavity consists of a pair of dielectric mirrors of spherical curvature mounted to face each other at a distance equal to approximately 95 percent of their radius of curvature and adjusted so that their optical axes coincide. The amplifier tube is positioned coaxially between the mirrors.

When an electric field of sufficient potential to ionize the gas is applied to the electrodes, excited atoms of helium collide with and transfer energy to the neon atoms, raising the neon atoms to one or another of their higher energy levels. Subsequently the neon atoms spontaneously drop to one or another of the lower energy levels they naturally occupy and simultaneously emit light of the wavelength that is characteristic of the energy released. Some transitions occur between the levels that give rise to emission at the wavelength of 6,328 angstroms.

Some photons of this wavelength are emitted along the axis of the tube. The energy then oscillates between the mirrors. During each transit through the tube this oscillating energy stimulates neon atoms that happen to occupy the appropriate energy level to drop to the appropriate lower level and thus contribute their energy to the resonator.

In lasers of this type the intensity of the oscillating light can increase as much as 5 percent during each transit through the amplifying tube. Although more than 500 million transits are made each second, energy stored in the resonator does not increase without limit. The efficiency of the amplifying action decreases as the stored energy increases. Efficiency is also impaired by the presence of impurities in the gases, departures from optimum gas pressure and changes in the ratio of helium to neon. In addition the stored energy is dissipated in various ways. Some is scattered by dust and imperfections on the surfaces of the mirrors and windows. Another portion is diverted by reflection from the surfaces of the windows. Ultimately less than two-tenths of percent seeps through the mirrors. This small fraction constitutes the useful output of the laser.

The amplifying tube of the first apparatus I constructed operated only 15 hours before failing. That the laser worked at all was gratifying, of course, but I felt that I was entitled to a longer run for my pains. The immediate cause of failure was "sputtering," which is the erosion of the metal electrodes by the electrified gases. Metal thus eroded collects in part as a film on the glass walls and lowers the pressure of the gas by burying atoms of helium and neon in the debris. The lowered pressure accelerates the phenomenon, which soon leads to the destruction of the tube.

Some metals tend to sputter more readily than others. The destructive action can be minimized by coating electrodes with a metal such as barium that increases conductivity in the vicinity of the electrode, thereby lowering the velocity of the ions in the plasma and the consequences of their impact on the metal. Coated electrodes must be purged of the occluded gases when the tube is constructed. This is usually done by operating the tube for a time on current intense enough to heat the meta I to redness. I omitted the step during my initial construction in an attempt to minimize the cost of the project. By trial and error I found that electrodes of aluminum foil resist erosion without having to be heated. The oxide coating naturally present on aluminum appears to retard the sputtering action for as long as 50 hours of operation if all other conditions are favorable.

Sputtering appears to be accelerated by a trace of almost any organic vapor. This became evident from the operation of the manometer I used originally for measuring gas pressure. The instrument employed phthalate as the indicating fluid. The vapor passed readily through a trap refrigerated by a slurry of dry ice and acetone and eventually contaminated the entire vacuum system. From then on even aluminum electrodes sputtered severely after an hour of operation, and no tube gave as many as five hours of service.

The difficulty was overcome by substituting for the manometer a gauge of the McLeod type, which uses mercury as the indicating fluid. The version of the gauge that I made consists of three joined capillary tubes, each of which contains a bulb [see Figure 5 ]. One bulb terminates in a closed capillary about 7.5 centimeters long. The middle leg contains a small spherical bulb that is connected to the vacuum system by means of a coil of copper tubing. The third leg terminates in a bulb substantially larger than the other two.

The glass structure is supported on a fixture improvised of plywood and pipe fittings that allows the assembly to be rotated through an angle of about 100 degrees for transferring the mercury by gravity from the largest bulb, in which it is stored, to the other two legs of the gauge. Normally the gauge is kept in the standby position with the mercury in the large bulb. To make a reading the assembly is rotated to the upright position This causes the mercury to run into the other legs. A specimen of the gas under measurement is trapped and compressed in the closed capillary, limiting the height to which the mercury can rise in the tube.

When the system is fully exhausted, mercury rises to the top of the closed capillary, indicating zero pressure. Simultaneously the metal rises to the same level in the bulb of the middle leg as well as in the reservoir. When the system is not fully exhausted, compressed gas prevents the mercury from rising to the end of the closed capillary. The distance between the closed end and the meniscus, or curved top, of the mercury is a measure of the pressure. In effect the gauge acts as a closed-end manometer.

A vertical scale, plotted in torr, is adjacent to the closed capillary, the zero graduation coinciding with the closed end. Graduations representing higher pressures are plotted at appropriate distances below the zero graduation. Only two quantities must be determined to compute the locations of the graduations: (1) the volume of the closed capillary plus the volume of the bulb to which it is attached and the volume of the capillary that connects this bulb to the middle leg of the gauge, (2) the cross-sectional area of the closed capillary.

To measure the volume, first weigh the glassware (to a tenth of a gram), then fill the volume to be determined with mercury and weigh it again. Subtract the weight of the glass from that of the glass and the mercury combined to determine the net weight of the mercury. Divide the net weight of the mercury by .0135 to determine the volume of the glassware in cubic millimeters. The cross-sectional area of a two-millimeter capillary is approximately three square millimeters.

To determine the distance in millimeters at which mercury will stand below the zero graduation for any pressure first multiply the volume just measured by the given pressure. Then divide this product by the cross-sectional area of the closed capillary. The square root of this quotient is equal to the distance in millimeters. For example, assume that the position of the graduation is desired for a pressure of 1.5 torr, that the volume of the closed capillary and its associated bulb and connecting tubing is 4,500 cubic millimeters and that the cross-sectional area of the. closed capillary is three square millimeters. The distance between the zero graduation and the desired graduation is then equal to the square root of 1.5 x 4,500/3, or 47.4 millimeters. To compute the entire scale, make a list of selected pressure intervals, such as .1, .5, 1, 1.5 and so on, and do the arithmetic. Plot the resulting distances in millimeters as graduations on a cardboard scale and cement it to the closed capillary with the zero indication adjacent to the closed end. If the volume of the closed capillary, its associated bulb and connecting tubing is approximately 4,500 cubic millimeters, a 75-millimeter scale will span the range of pressure from zero to three torr. Helium-neon lasers operate within this range.

The useful life of the amplifier tube is also reduced by the release of gases naturally present in the metal of the electrodes. Such gases can be dislodged by repeatedly filling the tube with the helium-neon mixture to a pressure of a torr or so and energizing it with an alternating current of 18 milliamperes, the rated output of the neon-sign transformer from which the tube operates. This is a time-consuming business, however. I now use direct current at 120 milliamperes for heating, and thus driving the gas out of, electrodes of metals other than aluminum. The current is supplied by a General Electric constantcurrent transformer (Model 916Y11) that develops 15,000 volts at 60 milliamperes. The unit is energized from the 120-volt power line through a variablevoltage transformer. The secondary of the constant-current transformer is tapped at the center of the secondary winding and grounded to the case by the manufacturer. The output of the secondary is converted to direct current by a conventional full-wave rectifier circuit that employs silicon diodes.

To outgas the electrodes the amplifier is first filled with about three torr of the helium-neon mixture. Power is then applied. The discharge current is gradually increased from minimum to 120 milliamperes during an interval of about 30 seconds. Ion bombardment heats the cathode to redness and the released gases change the color of the plasma from red to blue. Power is then shut off and the tube is pumped down. The procedure is repeated until the electrode is fully outgassed, that is, until the plasma retains its red color. The polarity is reversed for similarly processing the second electrode. Getters are assembled in a sidearm that serves as a reservoir for stabilizing the pressure of the gas. The glass of the reservoir and inner wall of the envelope of the amplifier tube is outgassed by initiating a discharge on alternating current at 18 milliamperes between the getters and the electrodes.

I am now experimenting with iron electrodes in the form of a cylinder, the inner wall of which is coated with barium strontium carbonate, and also with similar electrodes of titanium foil. The titanium is coated by evaporating barium inside the cylinder from a conventional getter of the KIC type. Tubes with coated electrodes of iron have operated from 50 hours to more than 100. Thereafter they can be reconditioned. (Coated-iron electrodes, barium getters, McLeod gauges and other supplies for experimenting with helium-neon lasers can be procured from Henry Prescott, 150 Main Street, Northfield, Mass. 02118.)

To open a spent amplifying tube for replacing the gas or for some other repair, crack the filling tube by heating the tip of the glass in a sharp flame. After an hour or two, when the amplifier has reached atmospheric pressure, the cracked tip can be removed. The slow leak prevents an inrush of air from depositing dust on the windows. If electrodes are also replaced, the glass blower must insert a desiccator in the blow hose to prevent moisture from condensing on the windows. I use an air filter charged with anhydrous calcium sulfate. Smudged windows seriously reduce the output.

A simple procedure for aligning the mirrors of the resonator has been described in the American Journal of Physics for March by K. L. Vander Sluis, G. K. Werner, P. M. Griffin, H. W. Morgan, O. B. Rudolph and P. A. Staats. The alignment tool consists of a square of white cardboard of any convenient size that is blackened on one side. A pair of fine diagonal lines are ruled on the white side of the card, which is pierced with a half-millimeter pinhole at the point where the lines intersect. A quarter-inch square of red gelatin (roughly the color of an Eastman Wratten No. 29 filter) is placed over the pinhole on the black side and cemented in place at the edges.

To align the resonator a small sheet of glass, such as a microscope slide, is placed at approximately right angles to the axis of the tube between one mirror and its facing window. This "spoiler" glass prevents the laser from generating a beam. The experimenter now energizes the amplifier tube and, from the blackened side of the card, looks through the pinhole and either of the mirrors into the capillary of the amplifier tube. The position of the eye is changed until the distant end of the capillary appears as a concentric circle inside the larger circle, which is the near end of the capillary. While the eye is in this position the adjustment screws of the mirror cell are manipulated until the image of the crossed lines, which is reflected by the near side of the mirror, is brought precisely to the center of the inner circle. The second mirror is then similarly adjusted.

The screws are now carefully rocked a degree or so in each direction. At some point a crescent of bright light will appear at the edge of the inner circle, resembling the rising moon. The screws are then gradually manipulated to produce a full moon. The second mirror is similarly adjusted for the full-moon condition. When the spoiler glass is removed, the apparatus will "lase." The adjustment is now trimmed for maximum output. Caution: Never adjust the mirrors by this method unless the spoiler glass is in position. The beam may form and permanently damage the experimenter's eye.

I have recently repeated a fascinating experiment with the laser that was described in the American Journal of Physics for May, 1964, by David Dutton, M. Parker Givens and Robert Hopkins. The experiment suggests a number of applications for the apparatus, among them the detection of microscopic movements of an object at distances up to 100 feet. The beam is first collimated by a microscope objective with a focal length of approximately eight millimeters; it projects the rays to a lens of two-centimeter aperture and proportionately longer focal length. The collimated beam is then directed into a Michelson interferometer, the beam splitter of which can be an unsilvered microscope slide. One portion of the split beam proceeds a few centimeters to a plane front-surfaced mirror of optical quality; the other portion, which is transmitted at right angles by the beam splitter, is projected to a distant mirror of the cube-corner type.

Both the plane and the corner mirror are adjusted to return the reflected rays to the beam splitter, where they combine and interfere. The interference fringes are picked up by a photocell that acts as the input to an amplifier and a loudspeaker [see Figure 9]. When the corner mirror is moved at velocities of less than approximately one centimeter per second, the fringes modulate the photocell and a tone is emitted from the loudspeaker that varies in pitch in proportion to the velocity. The loudspeaker emits a rumbling sound even when the corner mirror is apparently at rest. This is explained by random vibrations, including microseisms, as well as by variations in the refractive index of the air. Numerous possible applications come to mind for the apparatus, including strain seismographs, the precise determination of length and the accurate monitoring of distant positions.

Bibliography

EXPERIMENTS IN PHYSICAL OPTICS USING CONTINUOUS LASER LIGHT. T. J. Perkins. Optics Technology, Inc., 1964.

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