STAR MAGNITUDES AND IMAGE DIAMETERS IN PHOTOGRAPHIC PHOTOMETRY

Abstract

FORMULAE hitherto employed to determine the JL relationship between star magnitudes and image diameters on photographic plates have been empirical, and fail in certain circumstances. It is impossible to apply them in the case of bright stars which give diameters larger than a certain limiting value (and this value varies with different formulae) and,v in addition, the formulae take no account of the effect of star colours. D. L. Edwards has described a new method of investigation which gives very satisfactory results (Mon. Not. Roy. Astro. Soc., 102, 5). The work was carried out at the Norman Lockyer Observatory, Sidmouth. Three different Zeiss triplet lenses were used : (a) aperture 14 cm., focal-length 70 cm., (6) aperture 10 '4 cm., focal-length 50 cm., (c) similar to (b) but stopped down to 2 cm. aperture. The plates used were the Barnet Super Press (blue sensitive emulsion), and Ilford Hypersensitive Panchromatic, which gives a good scale of photovisual magnitudes without a filter. When the panchromatic plates were considered and measured diameters were plotted against H. D. visual magnitudes (only stars of type AO being used to determine the form of the relation for one colour only) the empirical formula m = a-b(D-kD2) gave the best fit. In this formula m is the magnitude, D the diameter, and a, b, k are constants.