Sky & Telescope July 1981 [antikvár]

Improved Techniques for AstrophotographyDavid F. Malin, Anglo-Australian ObsetyatorySEVERAL RECENT ARTICLES in this magazine have described ways to improve the sensitivity of photographic materials for use at the low light levels encountered in astronomy. These hypersensitizing procedures have been in widespread use by professional astronomers for several years, and are now an essential part of photographic technique. At the Anglo-Austra-lian Observatory we routinely hypersensi-tize all plates exposed with the 3.9-meter telescope. The United Kingdom 1.2-meter Schmidt telescope has almost completed a deep survey of the southern sky, with each of the more than 600 fields being photographed on hypered plates. The new generation of fine-grain, high-contrast emulsions such as Eastman Kodak Illa-J and IIla-F would be much less effective without hypersensitization. and a deep-sky survey would be almost impossible.Unfortunately, our highly sensitized plates are capable of registering light fromsources quite apart from the distant stars and galaxies. But where does this light come from? Surely, in a dark place such as Siding Spring Mountain, there are no streetlights. Away from the cities. Australia's night sky is as dark as anywhere in the world. But consider that when the Sun is up the sky is not dark. The sky is a beautiful blue, due to sunlight scattered by particles in our atmosphere. At night the air scatters starlight enough to slowly fog our photographs.Other contributions to the night sky brightness come from our nearest star in a different way. Energetic particles, traveling out from the Sun much more slowly than visible light, interact with our atmosphere and cause it to glow faintly. An extreme example of this, seen especially when the Sun is active, are the auroras flickering curtains of color usually visible from high latitudes in both hemispheres. Even when the Sun is quiet, electrically charged parti-cles bombard our atmosphere day and night and produce a faint light called air-glow. Other sources of unwanted illumination are sunlight scattered by the dust in our solar system (the zodiacal light) and starlight scattered by dust in our galaxy. Thus, even on the darkest nights, the sky is never truly black. Photographic exposures with a large telescope are limited not by the time available, but by the persistent fogging.As hypersensitizing techniques become generally available, users of modest equipment, whose image quality is influenced more by photographic material than by seeing, will turn to fine-grain emulsions. It is fortunate that these products respond so well to gas hypering, much better, in fact, than their inherently faster but coarsegrained counterparts. New converts to these superior materials will undoubtedly encounter some of the difficulties as well as the advantages they confer. This article describes simple darkroom techniques whichThe magnificent Orion nebula as recorded in a five-minuie red-light exposure with the 3.9-meter Anglo-Australian telescope (AAT). The original plate has been copied with the unsharp masking technique described in the text. This allows holding detail in the bright central region around the Trapezium stars without losing the much fainter outlying filaments. All photographs are from the author. 1979 Anglo-Australian Telescope Board.4 Sky and Telescope. July. 1981