Photographic amplification of faint astronomical images

Abstract

METHODS of contrast enhancement in astronomy, such as optical contrasting¹ or optical printing onto a high-contrast material, include all grains throughout the emulsion thickness, and noise (fog), as well as the image, is enhanced. Fairly high levels of chemical fog (up to diffuse densities of 0.3) are common when plates are optimally hypersensitised by baking or prolonged soaking in nitrogen followed by reduction sensitisation in hydrogen gas. Shaw² has shown that chemical fog seriously affects the detective quantum efficiency (DQE) of an emulsion when all the fog grains appear in the image as additional noise. I describe here a method that has been devised to make visible extremely faint images on astronomical plates without imaging fog grains. The images are not apparent on visual inspection and cannot be detected by conventional macro-densitometry. These effects result in a significant improvement in the visual detection of faint images on the original plate over the density range occupied by threshold imagery. The technique is most useful for revealing faint nebulosity and low surface brightness features on fine-grain high-contrast emulsions such as Kodak spectroscopic plates type IIIa.