Abstract
ABSOLUTE SCALES OF PHOTOGRAPHIC AND PHOTO-VISUAL MAGNITUDES.—A great piece of photometric work on which the 60-in. reflector of the Mount Wilson Observatory is engaged is the determination of absolute scales of photographic and photovisual magnitudes covering the whole range from brightest to faintest known stars. An account of the present position of the investigation has been communicated by F. A. Seares to the National Academy of Sciences (U.S.A.), vol. i., p. 307, 1915. The method employed involves the comparison of two series of images with a known variation of intensity between the exposures. The many practical difficulties have been successfully overcome at Mount Wilson by the use of wire gauze screens and circular diaphragms. The photographic scale for the intermediate stars (10-18 mags.) was first determined in two series of exposures, one set of eleven minutes and less, the other thirty to sixty minutes; numerous determinations were made. The average difference between the mean scales from the two series, derived from nine groups of stars between 10-6 and 16-8, is only 0015 mag. The extension to the fainter objects was effected by plates which received two different exposures with the full aperture of 60 in., the longer exposures of four to five hours, the shorter approximately half an hour. The limiting photographic magnitude thus reached was about 20. The bright stars (brighter than 10 mag.) were photographed with screens or diaphragms interposed producing images comparable with those of stars between the tenth and fifteenth magnitude, obtained with the same exposure with unreduced aperture. The entire series of photographic magnitudes of 617 objects was reduced to the international zero point by making the mean brightness of white (Ao) stars near the sixth magnitude equal to the mean of their visual magnitudes in Harvard Circular 170.
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Our Astronomical Column . Nature 95, 654–655 (1915). https://doi.org/10.1038/095654a0
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DOI: https://doi.org/10.1038/095654a0