Astronomical Photographs by David Malin,
Anglo-Australian Observatory
The Techniques
Most of the color photographs in this catalogue are made with the 3.9m Anglo-Australian Telescope (AAT) or the UK Schmidt Telescopes, located on Siding Spring Mountain in New South Wales, Australia. When the AAT is used as a camera it has a focal length of 12.7 meters and a focal ratio of f/3.3, a very impressive reflecting 'lens'. The field of view covers 1 degree of sky (about twice the diameter of the full moon) and the images are recorded on special black and white photographic emulsions coated on glass plates 255 mm (10 inches) square. The UK Schmidt is primarily a photographic survey telescope and has a focal length of 3.07m with a focal ratio of F/2.5. It photographs a 6.6 x 6.6 degree field on glass plates 356 mm (14 inches) square.
The color photographs are made by photographing each scene three times, with combinations of plates and filters designed to record blue, green or red light, chosen so that all the colors in visible light are recorded. Before exposure, the plates are specially pre-treated (hypersensitized) to increase their sensitivity to the feeble light from distant stars and galaxies. This treatment usually consists of baking or soaking the plates in nitrogen gas for several hours in a moderate oven, followed by a room temperature soak in pure hydrogen. This has the effect of increasing the long-exposure speed by a large amount. Even so, exposures of 30-40 minutes are required in each of the three colors for some of the faintest objects.
The color pictures have been made by photographically combining the three separate, black and white images using techniques developed by David Malin in the photographic laboratories at the Anglo-Australian Observatory. Black and white positive film copies are made from each of the plates, each image containing either the red, green or blue information from the object being photographed. The positives are projected by an enlarger, one at a time, through red, green or blue filters and in perfect register, on to a sheet of color negative film. From this master negative, derivatives of any size can be made. This elaborate process is necessary because conventional color films do not have sufficient speed, contrast or spectral sensitivity for recording faint objects in their correct colors. However, it produces colors that are true-to-life.
Before being combined into a color picture, the black and white images on the glass negatives can be enhanced by special copying techniques to reveal extremely faint objects, while the enormous brightness range found in star forming regions can be adjusted by unsharp masks to make their internal structure visible. The effect of these procedures on the final color print is to give the impression that the color sensitivity of the eye has been greatly increased and that the objects are being seen in their natural colors. These techniques are fully explained in "The Colors of the Stars", a popular-level book by David Malin and Paul Murdin (Cambridge University Press, 1984), and in a series of articles in Eastman Kodak's Tech Bits magazine, Issues 1 and 3 of 1990. David Malin's book "A View of the Universe" (Cambridge University Press, 1993) describes and illustrates many more of these unique astronomical photographs.
Using a large telescope is much more than a one-person task. At the Anglo- Australian Telescope the telescope itself is always driven by a night assistant, and the act of 'observing' is physically demanding and often shared with colleagues, especially during long winter nights. At the UK Schmidt, a dedicated observing team take plates of superb quality on request, however, in both cases, it is in the darkroom that the images seen here are created from these raw materials, which are only possible with superbly crafted telescopes and skilled support staff. |
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