These colors first came to public attention 10 years ago when astronomers at the Mount Wilson and Palomar Observatories made a series of photographs with color film. Amateur astronomers enthusiastically responded to the pictures and promptly tried to duplicate them with homemade equipment. The results were disappointing: the film came out either blank or foggy. The Mount Wilson and Palomar pictures had been made by exposing high-speed film for periods up to four hours in the 200-inch telescope at a focal length of f/3.3 and in the 48-inch Schmidt telescope at /2.5. The combination of large aperture and long focal length is of course beyond the reach of amateurs.

Not all amateurs were discouraged. One who kept trying is J. R. Bruman of 3527 Cody Road in Sherman Oaks, Calif. Last year Bruman, adapting a technique devised by Arthur A. Hoag, an astronomer at the Kitt Peak National Observatory, worked out a simple method of making good color photographs of the night sky with ordinary roll film in a camera equipped with an inexpensive refrigeration unit that he made at home. Several of his photographs appear above. Bruman writes:

"If we received more light from space, the problem of photographing deep-sky objects in color would be simple. The light that enters the telescope ranges from a ten-thousandth to a millionth of the brightness of an average daylight scene. When we make photographs in daylight, we compensate for dim light by increasing the exposure, perhaps even switching from snapshots to time exposures. The reciprocal relation between available light and exposure time is known to everyone who owns a camera.

"When the illumination falls to the point where the required exposure exceeds a minute or so, strange things begin to happen in the film. The reciprocity rule breaks down, and eventually, as one attempts to make pictures with less and less light, a time comes when the film is either blank or seriously underexposed, no matter how long the shutter remains open. Deep-sky objects are in this range of illumination. They cannot be photographed with ordinary color film employed in the ordinary way. Other complications also arise. Even if one manages to obtain an image by extended exposure, the colors come out wrong, because the color sensitivity of each of the several layers of color emulsion in the film changes uniquely with the intensity of the illumination.

"What accounts for the failure of the reciprocity rule? Specialists explain that the latent image created when photons dissociate silver halide molecules in the film is not necessarily permanent. If the dissociation is weak as the result of a scant supply of photons, the ions tend to drift back together at about the same rate at which they are formed. Continued exposure at this level of illumination results in no net gain in ions.

"As one expert has explained it, forming a latent image on film is something like filling a leaky bucket with hot molasses. Pour in the thin molasses quickly and you can fill the bucket, but if you pour slowly, the thin fluid refuses to accumulate. If you switch to cold molasses, the thick fluid leaks at a much lower rate. You can fill the bucket with cold molasses by pouring at a much lower rate. The analogy applies to photographic emulsions. Some years ago it was discovered that if an emulsion is refrigerated during exposure, it will accumulate a latent image even under very dim light.

"At about the time the Mount Wilson and Palomar pictures appeared Hoag was experimenting with Ektachrome film in this way and getting excellent results. Hoag's refrigeration device consisted of a metal plate, just behind the film, that was cooled by various means, including dry ice. There was a difficulty, however, because fog and frost developed inside the camera as the moisture-laden air came in contact with the cold emulsion. To prevent this Hoag and others enclosed the film in a sealed compartment that had a glass window; they evacuated the compartment with a laboratory vacuum pump. The scheme worked but was awkward: the film had to be loaded one sheet at a time, and the need for a vacuum pump ruled out the technique for use in the field, at least for amateurs.

"A couple of years ago the notion occurred to me that if I could cool the film from the front by a flow of cold dry gas, I might not only overcome the problem of frosting without the use of a vacuum pump but also could substitute roll film, in a magazine, for the single sheets of cut film. To try the idea I made a well-insulated box for the film magazine and let the vapor from dry ice into the chamber. The resulting fog of carbon dioxide obscured the image. I added a fan to draw air across the fi]m, but the film did not get cold enough. Therefore I substituted liquid nitrogen in the dry-ice container. The cold gas chilled the film enough, but the arrangement consumed nitrogen at a rate that required an inconveniently large reservoir of liquid nitrogen. Next I tried using the boil-off of liquid nitrogen from a Dewar vacuum flask that was connected to the film chamber by a long, flexible tube. I was unable to insulate the hose well enough to provide adequate cooling in the film chamber without consuming liquid nitrogen at an unreasonable rate.

"Thus I arrived at the present configuration: the container of liquid nitrogen is attached to the camera [see Figure 7]. A 15-watt heater immersed in the reservoir boils off liquid nitrogen at a rate of about 500 milliliters per hour. The gas flows from the reservoir through a short tube, swirls across the film and escapes through the telescope. I feared that the resulting currents and variations in the temperature of the gas would distort the image, but no trouble developed. The only difficulty arose when I tried to advance the film. At the low temperature it became brittle and snapped. Now I use two magazines. After an exposure I remove the cold magazine and quickly wrap it in a plastic bag for 15 minutes to prevent frost from forming. After the film warms up it can be handled normally. On the whole the technique appears to be adequate. I am getting about the same improvement in film performance that Hoag reported.

"Liquid nitrogen is not particularly hazardous, but several precautions should be observed. Never put the liquid, even briefly, in a tightly stoppered container. Avoid the use of containers that become brittle at cryogenic temperatures. Styrofoam (or its equivalent) is a satisfactory material for use at low temperature. Make sure that the liberated gas does not displace the oxygen you require for breathing. This possibility is remote in the case of telescopes operated outside the observatory. All cryogenic liquids can cause serious injury if they are splashed in the eyes or on the skin. Do not handle them carelessly.

"Liquid nitrogen costs only a few cents per liter when it is bought in large quantities. Generally it is more widely available than dry ice. In most communities with a population of more than 10,000 one can find suppliers who stock liquid nitrogen to meet the routine demand of machine shops, numerous small industries and laboratories of all kinds. The technique of making color photographs of the sky with cooled emulsion can be undertaken by any resourceful person, certainly by amateur astronomers who have done guided celestial photography with black-and-white film or plates.

"At about the time I became interested in color photography I was a guest at the Table Mountain Observatory of the Jet Propulsion Laboratory near Los Angeles. The observatory is devoted to planetary studies in connection with the program of the National Aeronautics and Space Administration, but the main telescope is a general-purpose instrument of the Cassegrain type with a 24-inch aperture and a focal ratio of f/16. The color photographs that accompany this discussion [top] were made either with this instrument, with a war-surplus aerial camera lens of 36-inch focal length or with the 30-millimeter lens of my reflex camera.

"Specialists will have no difficulty pointing out technical flaws, but the pictures demonstrate what can be accomplished with film bought in a retail camera shop. For example, the photograph of the Crab Nebula was made on Ektachrome EH, which has an ASA rating of 160. The exposure was for one hour at a film temperature of -60 degrees Celsius. Development, a few hours later, was normal except for an increase of first developer time from seven minutes to 15 minutes. Forcing development in this way raises the effective ASA rating of Ektachrome EH to about 600, as I determined by making exposures of Jupiter at .5 second at f/112.

"It is also possible to enhance the results slightly by baking the film before use. I baked the Ektachrome used for the Crab Nebula for 24 hours at 50 degrees C. immediately before making the exposure. The instantaneous speed of the film appears to be unaffected, but its response to long exposure at low light intensity is somewhat improved, extending the range of subjects that can be tackled with forced development alone. Making film beforehand is much less effective than cooling during the exposure, so that I have given it up.

"Incidentally, amateurs who have never made photographs of the sky in black and white are urged to acquire a bit of experience before attempting to work with color, so that they will have at least a casual acquaintance with the location of various celestial objects, the principles of an equatorial clock drive and so on. To make successful photographs the telescope must automatically track the field where the desired object appears; in addition a selected star within the field must be kept centered in a reticle manually by means of auxiliary drives. Amateurs usually use a small separate telescope for observing the selected guide star, but professionals prefer to guide on part of the image that is being photographed, a method that eliminates ' possible error arising from mechanical flexure between the principal objective and the small guiding telescope. A prism at the edge of the field diverts a small part of the image into the eyepiece used for guiding.

"For this purpose I use a cheap six-millimeter orthoscopic eyepiece obtained from the Edmund Scientific Co., Barrington, N.J. 08007, to which I added a cross hair that is illuminated by a miniature neon lamp. The cross hair consists of two strands from a piece of glass cloth; they appear to be finer than the strands of a spider web and are much stronger. The six-millimeter eyepiece magnifies the image of the guiding star and its motion 40 diameters, which is about the optimum. The brightness of the neon lamp can be adjusted by a one-megohm radio volume-control unit that acts as a voltage divider. Commercial eyepieces of this type employ midget incandescent lamps, which are easier to control but require special power supplies. The light from lamps of either type must be kept dim so that it will not mask the image of the guide star.

"The controls that regulate the movement of the telescope may be either electrical or mechanical, but they must be designed for operation without removing the eye from the eyepiece. It is not easy to find and center a guide star in the small field of view. I first locate the guide star by examining on the ground glass the entire field to be photographed. This is done with the aid of a large magnifier. A dim light source, consisting of a piece of 1/16-inch brass tubing illuminated at one end by a second neon lamp, indicates on the ground glass the position of the small prism that diverts part of the image into the guiding eyepiece. It is easy to shift the selected guide star to this illuminated area, where it vanishes and immediately appears in the guiding eyepiece. Next I detach the reflex camera and finder, replacing it with the cooled-emulsion assembly.

"To avoid sky fog I limit exposures (in minutes) to approximately the square of the f ratio, for example 10 minutes at f/2.8, 60 minutes at f/8 and so on. The published magnitudes of deep-sky objects indicate the total amount of light, not the brightness, so that the appropriate time of exposure must be determined by trial and error. Generally I start by making the longest possible exposure, as determined by the f-ratio rule. Then I work downward and use forced development.

"The structural details of the apparatus were designed to be compatible with my 2 1/4-by-2 1/4-inch Hasselblad reflex camera, but the dimensions can be changed easily for cameras of other types Commercially available parts, such as the closeup extension tube that joins the spacer and the camera to the telescope (and also functions as a guiding microscope), were modified with hand tools for accepting the neon lamps and the small prism.

"Having made an exposure with color film at low temperature and developed the image, how certain can one be that the resulting colors are truly representative of the color emitted by the object? At the light levels involved in deep-sky photography one cannot see much color. Moreover, the sensation of color is subjective, so who is to say what an invisible color should be if it is intensified? The color values of astronomical subjects are not well known, and some specialists are dubious about the color rendition of the cooled-emulsion technique. Little quantitative information is available because exposure times far exceed those of ordinary photography and each film has its own distinct properties.

"To check the verity of the resulting colors I constructed a simple three-color sensitometer that enables me to make a series of photographs of a colored target under illumination that can be varied in intensity without change in color. From the resulting photographs the reciprocity factor of the film can be judged in order to detelmine its exposure requirements. The photographs also provide a basis for the selection of corrective filters. The illumination of common deep-sky objects about a millionth of ordinary daylight, so that the sensitometer was designed for this broad range.

"Essentially the sensitometer consists of a light-tight box, which supports a test pattern of three color filters in front of a camera lens, and a variable aperture through which the test pattern is illuminated. The camera is at one end of the box, the test target is in the middle and the variable aperture is at the other end [see Figure 8]. The largest aperture is simply the open end of the box, the smallest a hole .01 inch in diameter made in a piece of aluminum foil. The foil can be supported by a board that slides into a set of grooves in the box. The ratio of the area of the two openings is 360,000 to one, hence an exposure of .01 second through the large opening corresponds to an exposure of one hour through the small hole. An exposure of .01 second is well within the reciprocal range of commercial films, and an exposure of one hour is typical of time exposures made through the telescope.

"The back of the sensitometer is adapted to the front ring of the Hasselblad lens, which in turn fits either the cooled camera or the standard camera, so that long exposures can be made with refrigerated film or short ones with film at room temperature. Fractional-second exposures are made with the existing camera shutter, since they do not need to be cooled. The test pattern consists of three gelatin filters and a gray scale sandwiched between sheets of glass. To ensure uniform, diffused illumination across the pattern I use white paper over the variable aperture and opal glass as the front layer of the sandwich. The filters consist of a Wratten No. 47, blue; 58, green, and 25, red. I recommend this combination for making tests with Ektachrome because each filter transmits one of the three color ranges of the film without overlapping the other two. Therefore a test exposure made with the sensitometer will identify any layer of the film that may respond abnormally.

"Exposures have been made with the sensitometer using both sunlight and an incandescent lamp as the source. Both yield comparable results. One-hour cooled exposures made with the small aperture correspond closely, in terms of the results, to those made with the large aperture at .01 second. In every test using cooling the best match resulted when the illumination was exactly reciprocal to the exposure time, for the short exposure it was 360,000 times as intense as for the long exposure. In contrast, exposures made through the small aperture for one hour with uncooled film resulted in severe underexposure, equivalent to at least two stops, and the color balance could not be properly evaluated.

"The use of liquid-nitrogen cooling should open many new experimental opportunities to amateurs because the technique is both simple and inexpensive. The exposed films can be developed by local processing laboratories if they are equipped to do forced development on a custom basis. Although further experimentation is needed to determine quantitatively the response of films other than Ektachrome, I feel that the principle of cooling films on the emulsion side by nitrogen is now established."

Bibliography

EYE, FILM, AND CAMERA IN COLOR PHOTOGRAPHY. Ralph M. Evans. John Wiley & Sons, Inc., 1959.

EXPERIENCES WITH COOLED COLOR EMULSIONS. Arthur A. Hoag in Sky and Telescope, Vol. 28, No. 6, pages 332-334; December, 1966.

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