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29 August 2003

A Cold Camera for Astronomical Photography

by C. L. Stong
Excerpted from "Scientific American's The Amateur Scientist", first published December, 1973.

The basic technique of photographing dim celestial objects by chilling photosensitive emulsions was developed for astronomical photography by Arthur A. Hoag of the Kitt Peak National Observatory. The method is simple in principle but somewhat complex in application. The emulsion can be cooled to the required temperature simply by pressing the film against the side of a metal box that is tightly packed with pea-size lumps of dry ice. The trouble is that the cold film causes moisture in the air to condense as fog that fills the interior of the camera.

The problem is solved by exhausting air from the interior of the camera with a vacuum pump. Now, however, the camera must be placed inside an airtight vessel reinforced sufficiently to resist the crushing weight of the atmosphere. The external controls of the camera and some parts of the vessel tend to collect frost, but this problem can be solved by recourse to electric heating units. The resulting hot spots give rise to new complications that lead to still other links in a growing chain of compensating gadgetry that suggests the design of a modern automobile.

The cost of such cameras exceeded the budget of most amateur astronomers. That was the state of the art when Bill Williams of Mahwah, N.J. became interested in astronomy. He explains as follows how he solved the problem of making deep-sky photographs with limited funds.

"Like most beginners who develop an interest in astronomy, I tired rather quickly of looking at the moon and a dozen or so other 'easy' objects. After leafing through magazines that display spectacular deep-sky photographs, I decided to convert my telescope into a camera. The technical literature, however, was discouraging. It disclosed that the best astronomical photographs had been made by freezing the film in specially designed cameras that utilize vacuum chambers, actuating valves, O rings and liquefied gases.

"I could not help wondering why the film had to be so cold. The answer involved a property of photographic emulsions that I had not known about. Everyone who makes snapshots is familiar with the inverse relation between the speed of the camera shutter and the size of the lens opening For a given amount of light to strike the photosensitive emulsion either the shutter speed is fast and the lens opening small or the shutter speed is slow and the lens opening large. The predictable response of the emulsion to this relation is termed the reciprocity rule.

"I did not know that the reciprocity rule fails when the light becomes so dim that exposures must exceed more than two or three minutes. I had always assumed that the speed at which film is rated by the manufacturer is valid under all circumstances. For example, high-speed Ektachrome film is assigned a speed rating of ASA 160. I was astonished to learn that this rating applies only to exposures of about .01 second. The rating does not change substantially even for exposures of a few minutes, but when the shutter must be kept open for two hours, the speed of high-speed Ektachrome falls to ASA 5! Tri-X emulsion, which is rated at ASA 400 for making snapshots, falls below the rating of Plus-X (ASA 125) in the case of time exposures that exceed 30 minutes. I also learned that some slow films, such as Eastman Kodak's spectroscopic emulsions, resist reciprocity failure and that emulsions of this kind are relatively unaffected by refrigeration.

"The latent photographic image that is created when photons dissociate silver halide molecules of the emulsion is not necessarily permanent. Given sufficient time, the ions tend to drift back together again at about the same rate at which they are formed. Thereafter no net gain in exposure results no matter how long light falls on the film.

"Lowering the temperature of the emulsion tends to suppress the recombination of the ions and hence to suppress reciprocity failure. Although cooling lowers the intrinsic sensitivity of the film, it disproportionately suppresses reciprocity failure. The result is a substantial net gain in the speed of refrigerated film for exposures lasting more than about a minute.

"The net gain in speed also improves the resolution of deep-sky photographs by reducing the effects of guiding errors, which arise from the necessity of keeping the telescope pointed exactly at the celestial object as it apparently moves across the sky. The shorter exposure also reduces the interval during which the image jiggles as a consequence of atmospheric instability. It is much more likely that the atmosphere will be steady and that the observer can keep the cross hairs of the telescope centered on a star for 10 minutes rather than for an hour. Incidentally, I have observed that an accurately guided six-inch telescope makes far better photographs than a poorly guided 20-inch instrument.

"Cooling greatly reduces the required exposure intervals in the case of both black-and-white and color emulsions. It also improves the latitude of both kinds of film, meaning that it enables the emulsion to register dim features of the image without overexposing bright areas. In addition cooling restores the color balance that is ordinarily distorted by making long exposures in dim light. Color film consists of several layers of emulsion, each with a unique reciprocity characteristic. Cooling suppresses reciprocity failure disproportionately in the less sensitive layers of emulsion with the result that all colors register faithfully even in dim light. The effect is strikingly apparent in the first of my two photographs of the Orion nebula, which reproduces blues and greens that are not evident in the photograph made at ambient temperature.

"When I set out to make a conventional cold camera, I enlisted the help of a friend, Scott Usher. We tried to simplify the construction in a number of ways without much success until we observed that frost does not form on photographic film that is sandwiched tightly between flat surfaces chilled by subliming dry ice. The literature had given us the incorrect impression that the film had to be in a vacuum. Clearly all we needed was a transparent barrier of low heat conductivity. We got the idea of making a heat insulator in the form of a vacuum cell with opposing windows. We intended to press the film tightly against the external surface of one of the windows by a flat plate chilled to the required-75 degrees C.

"Our vacuum cell proved to be faulty. While we were considering how to correct the design, it occurred to us that transparent heat insulators need not be based on a vacuum. Why not fill the cell with alcohol or a comparable liquid that freezes at a very low temperature?

"It also occurred to us that a transparent solid of low heat conductivity would doubtless work as well as a liquid. For example, a cylindrical plug of glass with flat, polished ends might do the job. The film could be pressed tightly against one end of the cylinder by a metallic container of dry ice. Light would be admitted through the other end of the cylinder.

"The idea seemed promising. How long should the cylinder be so that frost would not form on one end until the other end had been cooled for a period of time greater than the longest exposure of interest? By experimenting we found that a plug about two inches long did not frost over for an hour.

"Next we learned that a clear plastic, such as methyl methacrylate (Plexiglas), can be substituted for glass if the ends of the cylinder are made reasonably flat, parallel and polished. We also wondered if a thick window would degrade the quality of the image. We knew that similarly thick plugs of glass have been used to correct spherical aberration of lenses of large aperture but short focal length. The focal ratio of our telescope was about f/8. For this reason we felt that any possible overcorrection of spherical aberration would be trivial.

"We also investigated the question of how much light might be absorbed by two inches of plastic. Normally about 4 percent is lost by reflection at each surface. Our experiments disclosed that methyl methacrylate transmits approximately 92 percent of white light. Glass transmits slightly less light, is a somewhat better heat insulator and is more scratch-resistant than plastic. It is also more difficult to cut and polish.

Cross section of the camera assembly. Click image to enlarge

"In its final form our camera consist of three principal subassemblies. W made the body of a thick-walled tube of opaque plastic, one end of which is externally threaded. The threads engage mating internal threads of a tubular bracket that attaches the camera to the telescope. Images of celestial objects are focused by screwing the body into or out of the mounting bracket. From the unthreaded end the internal wall of the body tube was bored to larger diameter to accept the plastic cylinder. The enlarged bore terminates in a shoulder somewhat more than halfway through the tube. The plastic cylinder rests against the shoulder [see illustration at right]. The outer surface of the plastic cylinder extends to within about 3/8 inch of the unthreaded end of the body tube.

"A saw kerf that extends approximately three-quarters of the way through the body tube at the level of the outer end of the plastic cylinder admits 35millimeter film to the camera. The film is pressed against the plastic by the flat bottom of a cylindrical box that contains lumps of dry ice. Pressure between the lumps of ice and the bottom of the box is maintained by a helical spring acting against a sliding disk that functions as the lid of the box. The box assembly is clamped to the housing by a retaining ring. Pressure between the bottom of the box and the film is maintained by stud bolts, which act through a pair of helical springs against the retaining ring.

"I use three interchangeable plastic cylinders. One is for focusing the telescope, an operation that subjects the plastic to abrasion and invites scratches. The other plugs, which are handled carefully to avoid scratches, are used alternately for making photographs. Frost forms on the cold plug immediately after it is removed from the camera. The plug warms and dries while the next exposure is made with the alternate plug.

 

Method of loading film. Click image to enlarge

"I have worked with both cut and roll film. Initially I made a circular cutter--a small, sharp blade attached to a cylinder of plastic that turned on an axial shaft. The bottom of the shaft carried a rubber foot. Placing the film on a flat support, I set the rubber foot in the center of the desired disk of film, lowered the cylinder on the shaft so that the cutting edge of the knife made contact with the plastic and then rotated the cylinder to cut out the disk.

"Recently I devised a simple technique for using roll film. I push the leader of a new roll of 35-millimeter film into an empty cassette, which acts as a light tight storage compartment. Exposures are made on the strip between the cassettes. The film has to be removed from the camera after each exposure to exchange the plastic plugs and refocus and reset the telescope. During this time I put the cold, brittle film and cassettes in a light-tight box to warm and dry. I mark one cassette with a piece of adhesive tape so that I can identify it by touch in the dark.

"The cold, brittle film between the cassettes must be handled gently until it warms. Frost collects on the emulsion and wets the film when it melts. Part of the moisture can be wiped off without damaging the emulsion by using conventional darkroom techniques. Exposed portions of the film are pushed into the storage cassette after they have warmed and dried. The film can be transferred between the light-tight storage box and the camera on moonless nights without risk of exposure if one stays away from man-made sources of light.

"To make a photograph I line up the telescope, adjust the clock drive and focus an object, such as a bright star, in the plane that will be occupied by the film. The focusing can be done in either of two ways. A conventional eyepiece can be fitted with a short tubelike spacer. The end of the spacer lies in the focal plane of the image. To focus the camera place the end of the spacer on the surface of the plastic plug and screw the body of the camera in or out of the mounting bracket to the point at which the image of the star is smallest.

"The camera can be focused more accurately by the method used most frequently by professional astronomers. Place a dab of India ink, a razor blade or any similar sharp opaque edge on the surface of the plastic plug. Manipulate the telescope to the position at which the image of a bright star is bisected by the opaque edge.

 

Details of flap shutter. Click image to enlarge

"When the eye is placed close to the opaque edge, the observer sees a portion of a disk of light that represents the objective mirror. Part of the disk appears to be cut off by a dense, straight shadow. If the telescope is now moved so that the straightedge cuts deeper into the image of the star, the shadow will move either in the same direction as the telescope or in the opposite direction, depending on whether the plane of the straightedge lies inside or outside the focal plane of the telescope.

"Screw the camera into or out of the bracket to the point at which the entire disk of the objective mirror appears as a uniform pattern of light and shade that changes form but not position when the straightedge cuts deeper or less deep into the image. The camera is then exactly focused. Telescope makers will recognize this focusing technique as a variant of the familiar Foucault knife-edge test. Exposures are then made by opening a flap shutter [see illustration at left].

"I have patented the camera and granted an exclusive license for its manufacture and sale to Celestron Pacific of Torrance, Calif., but permission is hereby granted to amateurs to make a duplicate camera for their own private use. I have now worked with the camera for three years in California, Vermont, Florida and New Jersey. It is light, portable and easy to use. It enables anyone who owns a telescope that can be kept pointed toward a selected star to make deep-sky photographs of professional quality."