"This project," writes Aime, "would never have reached the drawing board 15 years ago had I then realized its full proportions. Like all stargazers, I had long wanted a good telescope; in the 1920's I had acquired a few acres of inexpensive mountainside in Putnam County, 50 miles from the lights of New York, with the hazy notion that the plot would make a good site for an observatory. Until the end of World War II, however, I did most of my 'observing' the city, at the Hayden Planetarium of the American Museum of Natural History. Then I learned of the availability of the 16-inch Pyrex blanks. My close friend the late Albert G. Ingalls, who at that time conducted in Scientific American a department largely devoted to amateur telescope making, warned me that a 16-inch instrument was no undertaking for a beginner. 'If you are determined make a big telescope,' he said, 'first construct at least two small ones. You can make a six-inch, a 10-inch and a 16-inch in much less time than you will waste if you first tackle the 16-inch.' I ignored his sound advice. As a result I have spent more than 6,000 hours during past 11 years at the drill press and lathe alone, a figure that omits the machining time required by parts that were too large for the shop of the Hayden Planetarium, where I did much of the work.

"Other thousands of hours have gone into such related tasks as procuring materials and erecting the telescope. One of the largest parts, the hour circle, once functioned as the 1,500-pound flywheel of a rock-crusher. It was given to me for the taking by the owner of a gravel bank -but taking it involved the use of a 50-ton hydraulic jack and a husky friend. We managed to remove the wheel from its shaft and load it onto a truck in less than two hours. The smaller parts include odds and ends picked up from scrap piles all over the northern half the hemisphere. A special electrical fitting, for example, was recovered from the gutter of the Avenida Juarez in Mexico City. Luck at the scrap piles, in fact, played no little part in the final choice naterials.

"In its present form the telescope is of the Cassegrainian type: a perforated paraboloidal mirror of f/4 aperture directs converging rays of starlight to a convex mirror on the optical axis that in turn reflects the rays back through the perforation to a focal plane behind the objective mirror. The image can be examined through an eyepiece or photographed. The arrangement enables the designer to fit a relatively long optical path into a short mechanical structure and still achieve the high magnification made possible by extended focal length without sacrificing the convenience of a short instrument. The focal length of objective mirror is 64 inches. Starlight, after reflection by the secondary convex mirror, comes to focus at the optical equivalent of 256 inches. Objects examined through an eyepiece of one-inch focal length are magnified 256 diameters; objects viewed through an eyepiece with a focal length of six millimeters are magnified 1,085 diameters. At this maximum magnification the moon is seen at an apparent distance of about 230 miles and can be picked apart crater by crater.

The mechanical structure, or mounting, that supports the optical parts in physical alignment and keeps them trained on a desired star or other celestial object resembles the design proposed the late Russell W. Porter, who with Ingalls founded amateur telescope making as a hobby in the U.S. Essentially the structure consists of a skeleton tube suspended by bearings between the tines of a stiff fork that rotates about an axis parallel to that of the earth, as shown in the accompanying drawing [below]. The tube, in addition to supporting the mirrors and eyepiece, serves as the mounting for a 6.5-inch achromatic telescope of 96-inch focal length that is used for guiding the instrument when photographing stars. It is an excellent instrument in its own right, it resolves to Dawes's limit and presents a flat field substantially free of color. The tube also mounts two smaller achromatic telescopes of four and 4-3/16 inches respectively that serve as finders.

"The supporting fork is made of welded boiler plate and channel sections rigidly attached to one end of a solid steel shaft four inches in diameter that functions as the polar axis. The south end of the shaft rests in a thrust bearing that can be adjusted both laterally and vertically to point the shaft precisely toward the north celestial pole. The north end of the shaft makes a close fit with the center hole of the 1,500-pound flywheel, the rim of which rides on a pair of solid steel cylinders that are rotated by a clock mechanism. The cylinders and the thrust bearing thus form a three-point suspension, with the front points-the two cylinders-spread 15 degrees to the east and west of center. The entire instrument rests on a triangular pier of concrete that extends down to the solid rock of the mountain. The image is correspondingly rock-solid, even when the instrument is used at magnifications 36 per cent above the normally accepted limit of 50 diameters per inch of aperture.

"Although the tube and the movable parts of the mounting weigh more than 1 1/2 tons, the assembly can be rotated easily by hand in both right ascension and declination. For convenience and particularly for photographic observing, the tube is driven in both axes by electric motors. Sidereal drive is supplied by a 1/150-horsepower synchronous motor that operates from the 60-cycle power line. It carries the load easily without heating. The tube is advanced or retarded in right ascension by a second 1/150-horsepower motor geared to the drive mechanism through a differential. The drive mechanism also includes a gearshift for tracking either stars or the moon, and an electric clutch that allows the tube to be slued by hand. A universal motor drives the tube in declination through a train of gears at a rate that enables the observer to cut the moon's disk easily into eight parts. All three motors are controlled through a cable that terminates in a portable switch box at the eyepiece.

"The tube carries a platform just behind the cell of the objective mirror for supporting cameras, spectrographs, photometers and related accessories at the Cassegrainian focal plane. All instruments seat against stops in the platform so that they can be removed and then returned precisely to their former positions. I use either of two cameras as plateholders: a Leica M2 with or without its lens and a Medalist II. The Leica is focused by means of a microscope attachment and the Medalist is focused through a ground glass. When fitted with these plateholders, the optical assembly acts as a camera of substantial light-gathering power.

"The circular observatory is constructed of concrete blocks topped by an aluminum dome slightly more than 16 feet in diameter. The dome rolls on eight pulley wheels that ride a circular track of 8/4-inch pipe and is rotated by an electric winch controlled from the eyepiece position. Access to the sky is provided by a pair of quadrant shutters that roll laterally on similar tracks and are .moved by a hand winch. The clear area measures four feet in width and extends from the horizon to the zenith.

"In general the telescope is pleasant to use and capable of doing good work. On the other hand, several disadvantages should be mentioned. The optical elements are not easy to collimate, or align, and the field of view is quite narrow- only a tenth of the moon's diameter. This means that the optical axis of the finder telescope must be aligned within 1 1/2 minutes of arc with respect to the primary objective, and this was not an easy task. Until the finder was adjusted to the proper position I had difficulty in locating objects even as brilliant as Jupiter in the main scope, to say nothing of specific stars. The problem of pointing the polar axis directly toward the north celestial pole was even more difficult, in spite of jackscrews for shifting the south bearing laterally in small increments.

"Although the construction started as a one-man project, many friends were drafted for specific jobs. The figuring of the objective mirror, for example, was done by Stanley Brower, a former amateur turned professional. Friends also helped me to erect the observatory wall, weld the fork, lay the floor and pour the concrete pier. The heaviest machine work was done in a commercial shop.

"No amateur can expect to avoid errors in a project of this size, and I was no exception. I should not, for example, have geared the instrument directly to the motors for rotation in right ascension. I did not know much about observing when I designed the drive or I would have applied the power to the hour circle and linked the hour circle to the polar shaft by a clutch arrangement. The hour circle could then be set as a clock at the beginning of an observing session and the instrument thereafter clamped at any desired right ascension without the need for computing sidereal time when shifting from one celestial object to another. I am now reconstructing the drive to include this feature.

"Several scraps of knowledge emerged from the project that may prove useful to others who undertake the construction of an instrument with an aperture of more than six inches. First, decide kind of objects you want to observe and what kind of observing you want to before you attempt the design. Are you interested in visual observing or do you intend to go in for photography? Will the instrument be used primarily for observing the moon, planets and star clusters or individual stars or nebulae? No single instrument can perform equally well on objects of all classes.

"Second, complete the mechanical design before you cut or grind a single part, including the optics, the drive mechanism and the observatory. Nothing is more exasperating than to discover an omission that must somehow be squeezed into an otherwise complete assembly. Procure materials well in advance and make no component until its essential parts are at hand. Devote enough time and care to the job so that each piece reflects credit on your craftsmanship and is worthy of the instrument. Remember, you will be looking at the parts for years and they will be staring right back at you. Prepare a sketch of each part before you attempt to make it. Parts that seem ideal in the mind's eye have a way of becoming much simpler on the drawing board. Finally, if you do not know how to design a component, don't guess. Ask someone who knows."

According to Aime, the observance of these simple rules spared him many headaches, cut his construction time substantially and resulted in a better telescope. Now that Aime's dream has been fulfilled, what does he intend to do the instrument? I put the question to him a few weeks ago during a short session at the observatory.

"That's easy," he replied. "I have had a lot of fun playing with it already. Lunar detail interests me in particular and I am looking forward to having a ringside seat when the next spacecraft reaches the moon. I tried to observe the impact of the first Soviet moon probe but had no luck, perhaps because the instrument had not been properly collimated at that time. Not all my hobby hours are spent at the observatory, however. I enjoy working with my hands and for several years have taught mirror making to beginners at the Hayden Planetarium, where the Optical Division of the Amateur Astronomers Association of New York conducts evening courses for amateurs The observatory is made available on a scheduled basis without charge to graduate students in astronomy and other qualified observers."

I asked Aime what he knew about the remaining 96 Pyrex disks. He shrugged. The question is of more than passing interest because several school groups have indicated a desire to make large telescopes. Readers having knowledge of any 16-inch blanks in good condition can get in touch with these institutions through this department.

Most amateur photographers use light meters for determining correct exposures but rely on guesswork in the darkroom when they select printing paper of the proper contrast and the correct lens opening of the enlarging projector. George Ginn, an electronics engineer and amateur photographer of Mountain View, Calif., writes that he used to print by guess, "and after practicing for 10 years I became so proficient I could sometimes make a good black-and-white print in 80 minutes without wasting more than a dozen sheets of projection paper. Now I use a simple light meter that enables me to make a good print every time on the first try.

"There is nothing essentially new in the idea of controlling the process by measurement. Commercial laboratories long ago adopted densitometers for the purpose, but these devices cost substantially more than the average amateur can afford. My meter measure is not the density of the negative but the intensity of the light at the enlarger easel.

"I assembled my meter largely from parts found in the scrap box; it could be duplicated even with new parts for less than the cost of an ordinary exposure meter. It consists of a photocell connected by an extension cord to a few resistors, a regulated power supply and a microammeter. All parts except the photocell are housed in a small metal box.

"Proper exposure and paper contras t are fully determined by two measurements made by placing the photocell in the lightest and darkest parts of the negative image projected on the enlarging easel. The photocell does not generate current but acts as a resistance that varies inversely with the intensity of the impinging light. This property immediately suggests a circuit. The photocell, battery and microammeter can be connected in series, as shown by the first drawing of the accompanying set of circuit diagrams

[Figure 6]. This simple arrangement would work, but examination of the circuit discloses two obvious disadvantages. First, accidental exposure of the photocell to bright light would minimize the photocell resistance and the resulting high current could damage the microammeter. This defect can be corrected by inserting a limiting resistor in the circuit to keep the current to a safe value when the photocell is short-circuited, as shown in the second schematic diagram. The other disadvantage stems from a common property of all photocells: their resistance rises to maximum but does not become infinite in the absence of light. The current of constant minimum amplitude transmitted by photocells even in total darkness would deflect the needle of the microammeter to some point on the scale above zero, thereby wasting a portion of the scale. Most experimenters prefer light meters that indicate zero in darkness and full-scale deflection when exposed to light of maximum intensity. This can be achieved by two more modifications of the basic circuit. The first modification balances out the 'dark current' and the second provides an adjustment for controlling full-scale deflection. To balance out the dark current a variable balancing resistor is connected across the cell and microammeter and a fixed resistor across the microammeter and current-limiting resistor, as shown in the third diagram. The circuit configuration now constitutes a bridge, with the photocell and balancing resistor forming the upper branches of the bridge and the fixed resistor and current-limiting resistor forming the lower branches. The micro-ammeter connects across the center of the bridge between the upper and lower branches. If the resistance of the variable resistor is adjusted to equal that of the photocell in darkness and the resistance of the current-limiting resistor is selected to match that of the fixed resistor, the bridge will be balanced when the photocell is in darkness and each side will transmit equal current. No current will then appear in the microammeter. In effect, the dark current has been balanced out. The problem of adjusting the circuit for maximum meter deflection when maximum light falls on the photocell can be solved by installing a potentiometer across the battery for adjusting the voltage applied to the bridge circuit, as shown in the fourth diagram.

"A meter so constructed would work, but I wanted mine to operate from ordinary 110-volt, 60-cycle house current. So I added a rectifier and a small neon lamp that doubles as a light for the microammeter scale and as a regulator for maintaining the rectified voltage at constant amplitude. The final form of the instrument is shown in the fifth diagram.

"It turns out that the sensitivity of the meter is high at low values of light intensity and progressively less sensitive at higher intensities. This is an advantage because the measurement that determines exposure is made in the darkest area of the negative image, where maximum sensitivity is needed. A new scale was made for the meter and calibrated so that each scale division corresponds to one stop of the enlarger lens, or a factor of two in brightness. This conforms to the standard ASA scale used on exposure meters Each of the divisions was then subdivided into three parts, which represent a change in density of .1, as shown in the accompanying drawing [Figure 7].

"All components except the Clairex CL-4 photocell are assembled in a standard apparatus box five inches high, four inches wide and three inches deep. A window in the front cover provides access to the scale of the microammeter, which is mounted behind the front cover on tubular spacers and long bolts. The neon lamp is located behind the front cover and beneath the window to shield the cell from direct exposure to the lamp.

"To relate the light measurements to the characteristics of printing papers, I test each batch of paper as it is opened. For this purpose I made a test negative by masking a strip of process negative in increments and doubling the exposure of each increment to produce a 10-step gray scale. With this test strip in the enlarger, I determine for each paper the stop that causes a dark area of the negative to produce a barely discernible high-light gray in a print exposed for 15 seconds and a light area to produce a black just short of maximum. The next step is to take a meter reading, at the easel, of the light and dark areas of the negative image with the enlarger set at that stop. The difference between these two measurements is my contrast index for the paper in question. Depending on the individual meter, differences of 1, .8, .7, .6 and .5 may be found to characterize papers of contrast grades 0, 1, 2, 3 and 4 respectively.

"To print a negative, finally, insert it in the enlarger and open the lens until a meter reading in the darkest area of the negative image indicates that the high light will just register. Then take a reading in the lightest area. Subtract the lower reading from the higher one to find the difference between the two. Select a paper that matches this difference. A print made with this paper exposed at this lens opening for 15 minutes and developed as recommended by the manufacturer should come out right on the button."

Bibliography

AMATEUR TELESCOPE MAKING (BOOKS ONE, TWO AND THREE), edited by Albert G. Ingalls. Scientific American, Inc., 1926, 1937, 1953.

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