Sky & Telescope July 1983 [antikvár]

Color Photography from Mauna KeaLaird A. Thompson and Michael Pierce, Institute for Astronomy, University of HawaiiMauna kea observatory, located atop a 13,700-foot mountain on the largest of the Hawaiian Islands, is one of the best observing sites in the world. A large fraction of the nights are clear, and the thin air has excellent infrared transparency. But best of all, on many nights the "seeing" the sharpness of images viewed through a telescope is superb. Many astronomers at Mauna Kea take advantage of these periods of excellent seeing for a wide range of high-resolu-tion studies. But it also seemed appropriate to use a small part of the telescope time to produce, for the sake of public interest, a set of color photographs of deep-sky objects taken under these unusually fine conditions.The best telescope on Mauna Kea for a project of this type is the 3.6-meter Canada-France-Hawaii Telescope (CFHT). At prime focus it has a moderately fast f/4.2 focal ratio; telescopes with higher focal ratios would require excessively long exposure times with color film, which has relatively low sensitivity to light. Furthermore, the image scale at the CFHT prime focus is 13.6 arc seconds per millimeter. This is large enough so that even the smallest images produced by the telescope fall on many grains in the film, a condition necessary to record the finest details the telescope can resolve.Those who attempt color astrophotography face two practical problems. First, it is advisable to hypersensitize the film, because color film is designed for use in daylight, not the extremely low light levelsencountered in astronomy. Second, obtaining the correct color balance for astro-photographs is not at all straightforward. Careful thought must be given to what "correct color" actually means for an astronomical subject. Here is how we approached these problems in creating the photographs reproduced on these pages.HYPERSENSITIZING COLOR FILMIf color film is not hypersensitized, astronomical photographs either tend to be dismally underexposed or the exposure times are prohibitively long. For example, consider some early color photographs by astronomers those produced at Palomar Observatory with the 200-inch Hale telescope. These were made on slower film than is now available and without hypersensitizing. The famous Palomar color photograph of the Crab nebula took five hours to expose. In contrast, our photograph of the Crab took only 30 minutes, yet it was made with a slower optical system: f/4.2 instead of the Hale telescope's f/3.7 (with field correctors).**The difference in size between the two telescopes was not a factor. The exposure time for an extended object like the Crab nebula depends only on the f/ratio of a telescope, not its size. An amateur with a small f/4.2 telescope could use our procedures and exposure time to produce a photograph of the Crab just as deep as ours. The difference would be in the image scale. With a small optical system the Crab would be a tiny dot on the film, whereas our image of the nebula is 30 mm (IVa inches) across on the original film.Professional astronomers have been experimenting for well over 40 years with ways to hypersensitize photographic emulsions. The most successful involves heating, or baking, them in either pure nitrogen gas or nitrogen with a small admixture of hydrogen. The baking raises the likelihood that grains in the film will be activated if hit by light, while the gas leaches out water and oxygen from the film. When water and extraneous gases are present, the film's sensitivity drops.Word that such methods work extremely well on black-and-white emulsions inspired amateur astronomers to run similar tests on color film {see, for example. Sky & Telescope, February, 1981, page 174). To supplement these initial assessments, we performed more extensive tests with three Ektachrome transparency films: ASA 64, ASA 200, and ASA 400. Ektachrome was convenient because we were able to develop it ourselves soon after exposure. Our hypersensitizing trials are similar to those reported by the amateurs, though we used slightly more elaborate equipment.A sample film strip from our tests is on the bottom of the facing page; it shows a spectrum of white light. If Ektachrome could reproduce honestly each color (wavelength) of incoming light, the film strip would show a full, even rainbow of colors. Instead, we see three discrete color bands. Ultraviolet, violet, and blue light all record on the film as just one color royal blue. Green and yellow light both record as lime green, and orange and red light produce red.