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An instrument for measuring the photographic density (degree of blackening) at different points on a photographic emulsion or paper. For light passing through the emulsion, the transmitted density is measured; for prints the reflected density is measured. Modern computer-controlled measuring machines such as APM and SuperCOSMOS are sophisticated versions of the microdensitometer, designed to measure accurately the centres of star images and to distinguish stars from galaxies.

Subjects: Astronomy and Astrophysics.

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