The colour images are created by combining three separate, black and white exposures with combinations of plates and filters designed to record blue, green or red light. For the faintest objects exposures of 90 minutes are required for each colour. The monochromes are then manipulated, enhanced and combined photographically to produce the final picture, which show the object much as it might appear in the telescope if our eyes were much more sensitive to the colour of faint light. It is these techniques that are described here. Introduction The techniques developed by David Malin have led to a unified system of photography designed specifically for the unusual imaging requirements of astronomy.
Bright objects recorded on such materials are completly unprintable in any normal way.
However, the information in these high densities can be extracted by a technique which was initially used in the graphic arts industry. The blurring is usually achieved by placing the film for the unsharp mask in contact with the back of the glass negative and using a diffuse light source to make the exposure, the glass thickness of the plate acting as a spacer. The blurred positive cancels the low frequency information in the negative, acting as a kind of automatic dodging mask, leaving the fine detail untouched. This sounds more complicated than it is in practice, and a diagram should clarify the stages. Consistency, careful control of exposure and processing and a little experience are needed for success.