In July, 1850, W. C. Bond took what is believed to be the first astronomical photograph, on a wet collodion plate. Photography extended the range of observable magnitudes far beyond that of the eye, because in a time exposure a photographic plate can soak up light for hours on end, which the eye cannot.
Only a few stars are near enough, and big enough, that a telescope can image them as a disc.¹ Most are so far away that they appear as points, with no diameter; but on a photographic plate, the brighter the star, the bigger the image. Bond was also first to notice that the diameters of the stars' images appeared to be proportional to the logarithm of their brightness. Later workers have refined the relationship.
log (d + a) = e log (b + k log E)
d = diameter of disc in image
E = exposure of plate
a, b, k, e = constants
* determined empirically. See the discussion of the Greenwich, Ross and Scheiner formulas in
Frank Elmore Ross.
The Physics of the Developed Photographic Image.
Monographs on the Theory of Photography, no. 5.
New York: D. Van Nostrand; Rochester, NY: Eastman Kodak Co., 1924.
So magnitudes can be determined by measuring the diameters of star images on a photographic plate. Magnitudes that have been determined in this way are called photographic magnitudes. Most astronomical photographs are taken on types of film that are not equally sensitive to light of all colors. Blue magnitude or photovisual magnitude refers to a photographic magnitude that has been measured using photographic plates whose sensitivity to various colors is similar to that of the human eye.
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Last revised: 8 March 2008.