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The Jack Newton Cold Camera.

The cold camera's niche lies with astrophotographers who desire original transparencies rendered in the most accurate color that Ektachrome 400 can given.

ASTROPHOTOGRAPHERS have always placed unusual demands on their emulsions. Conventional film suffers from reciprocity failure, which is the emulsion's decreasing capacity to record faint light during long exposures. Accordingly, the low light levels encountered in deep-sky photography require disproportionately long exposures.

Recent methods to combat reciprocity failure have centered on gas hypersensitization. This process usually involves treating an emulsion in a mixture of nitrogen and hydrogen prior to exposure. Gas hypering is attractive because it allows using conventional camera equipment, while providing often dramatic increases in an emulsion's sensitivity during long exposures. But gas hypering also raises an emulsion's fog level, which is especially troubling with transparency films. Furthermore, gas-hypering color film can lead to shifts in the color balance.

A time-honored way of beating reciprocity failure is the cold camera. Cooling film to subzero temperatures during the exposure dramatically reduces exposure times without increasing fog or shifting color balance. Evered Kreimer used his homemade cold camera for the photographs in his now-classic Messier Album, which he authored with John Mallas. Celestron International marketed one of the few cold cameras ever produced commercially. Based on a design by amateur astronomer Bill Williams, the Celestron camera was discontinued about the time gas hypering began to catch the attention of amateur astrophotographers.

Fortunately, a commercial cold camera is again available. Renowned Canadian astrophotographer Jack Newton has designed one that offers distinct improvements over earlier models. Foremost is the ability to use 35-mm film in conventional cassettes rather than cut into individual frames. The Newton Cold Camera with its thin, smoothly operating shutter requires very little back focus, making it ideal for "fast" Newtonian reflectors with low-profile focusers.

The camera is simple in principle and design. During exposure, the film is sandwiched between an acrylic optical plug pressed against the emulsion and a metal plate chilled by dry ice. The plug prevents moisture-laden air from reaching the film and turning into frost. Made almost entirely of plastic, this camera is lightweight and built with carefully machined parts that fit together well.

Operating the camera is not so simple. To ensure an optimum exposure, you need to follow a checklist with more than 20 steps! Much of the routine involves the formidable task of chilling film to subzero temperatures while keeping it free of frost. Some of the protocols are unique to the Newton Cold Camera itself. If you follow the instructions exactly, you'll have a long evening at the telescope. Efficiency improves with practice, however, and spectacular photographs can make the effort worthwhile.

I tested the camera with my 12 1/2-inch f/4 Newtonian reflector. It slips into a standard 2-inch focuser, and an optional adapter ($18) fits the conventional T-thread equipment used with most Schmidt-Cassegrain telescopes. I like to use an off-axis guider rather than a separate scope, so I coupled the camera to a Lumicon Easy Guider with the T-thread adapter. This placed the guider ahead of the cold camera's shutter, which made easy work of centering a target star before opening the shutter.

At first I could not simultaneously focus the guider's eyepiece and camera. My solution was to make a 3/8-inch-long spacer from 1 1/2-inch plastic pipe. Fitted into the optical-plug assembly, this ring moved the plug (and camera's focus) farther back. Photographers working with separate guidescopes will not encounter this problem. Nevertheless, it would help if the instructions made some mention of it. Also, it would be nice to include a few exploded drawings of the camera showing various mounting arrangements.

The camera is focused on a moderately bright star using the knife-edge test. This involves looking through a clear plastic cap containing a coarse Ronchi grating held against the optical plug. If the cap were machined to a slightly smaller inside diameter it would fit snugly over the plug allowing "hands-free" operation that would make focusing a lot easier. This would be especially true in the case of refractors and Cassegrain telescopes that often require awkward viewing positions. It helps for users to be familiar with knife-edge focusing, since the instructions describe the method only in the most general terms.

It is important to keep the optical plug perfectly clean and scratch-free, since anything on the surface that contacts the film will show on the picture. I used a can of compressed air to clean the plug between exposures. Because the plug is plastic, it scratches easily. Replacements cost $19.95.

The plastic camera body has a metal "cold" plate that contacts the back of the film. After a pair of standard 35-mm film cassettes are placed in the camera, the film is advanced with a knob that turns only in the correct direction. Since a roll can be easily changed even though it isn't completely used, you can preload cassettes with various emulsions for use during the night.

Mounted on the back of the film holder is the dry-ice chamber with its screw-on cap and spring-loaded plunger for pressing the dry ice against the cold plate. The chamber holds about three tablespoonfuls of granular dry ice, which lasted about 40 minutes on a night when the air temperature was about 60|degrees~ Fahrenheit. The film remains quite cold for several minutes after the ice has completely sublimated.

The film holder slips onto the optical-plug assembly and is locked in position with a knurled thumbscrew. This operation must be carried out in darkness since the film would otherwise be exposed to light. It must also be done carefully because the film is sandwiched between the plug and cold plate, and any twisting motion could scratch the film and plug surface. Too much pressure may also shift the focus.

Charging the chamber with dry ice must be done after the camera and plug assembly have been locked together, otherwise frost forms on the film. The only dry ice available to me came in finger-size pellets sold by a medical supply store. I put them in a plastic bag and pulverized them with a mallet just prior to use.

The camera comes with a 1 1/4-inch eyepiece holder that fits in the place of the optical plug and is used for checking that the object to be photographed is centered in the field. It is supposed to be used after the camera is focused, but I found it more practical to compose my picture by holding a 2-inch, low-power eyepiece over the end of the plug. This eliminates removing the plug with the possibility of shifting the camera's focus.

After each exposure the camera body must be removed from the shutter assembly, its opening capped, and the dry ice emptied from the chamber. Cold film is very brittle, and the metal plate must be warmed before advancing the film. The instructions suggest using a portable hair dryer or car heater. It took almost three minutes with a 1,200-watt dryer to rid the plate of frost. When the camera body is removed the optical plug must also be heated to eliminate frost or condensation before the next exposure can begin. I used my car heater once! It took over 10 minutes -- after the car warmed up -- to remove all frost from just the camera body. Taking the shutter and optical plug into your car also means that you will likely need to refocus the telescope afterward.

In spite of all the preparation, the Newton Cold Camera works. I made test exposures with Kodak Tri-X and Ektachrome 400 -- two emulsions with proven cold-camera track records. Chilled Tri-X seemed to reach the sky-fog limit faster than gas-hypered Technical Pan 2415, though Tri-X had less resolution and contrast. Chilled Ektachrome showed no shift in color, just an overall increase in sensitivity.

The cold camera is definitely not a product for beginning deep-sky photographers. This one is designed by an experienced astrophotographer for experienced astrophotographers. When the camera's complexities are weighed against working with gas-hypered film, I suspect many photographers will prefer hypering. Even though chilled Tri-X film offers an extremely effective way to cut exposure times, I still like the resolution and contrast of Kodak's Technical Pan emulsion, which reportedly responds better to hypering than cooling.

The cold camera's niche lies with astrophotographers who desire original transparencies rendered in the most accurate color that Ektachrome 400 can give. The camera brings out the best in this film, since it holds color balance while providing sufficient sensitivity for recording faint deep-sky objects. Furthermore, this increased sensitivity is enough to allow exposures through light-pollution filters.

Most of my criticisms pertain to cold cameras in general and not to Newton's design. After about a dozen exposures, and becoming more proficient after each one, I'm convinced that the Newton Cold Camera is about as user-friendly as one of these devices can ever be. Considering the excellent craftsmanship, it's a bargain.

An accomplished amateur astronomer, Johnny Horne spends his daylight hours as a newspaper photographer for the Fayetteville Observer-Times.
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Title Annotation:S&T Test Report; camera for astronomical photography
Author:Horne, Johnny
Publication:Sky & Telescope
Article Type:Evaluation
Date:Mar 1, 1993
Previous Article:Explanatory Supplement to the Astronomical Almanac.
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