Talk:Instrumental magnitude

Incomplete equation
The equation for instrumental magnitude is not complete; there needs to be a constant on the right side, viz M = -2.5log(f) + K The constant is arbitrary when doing differential photometry, but otherwise is useful in accounting for the units of measureing flux. For example, depending on the imaging system gain, the number of photons per electron is variable, and thus the measured flux f, in ADU, is not the same as the number of photons detected. [References for this note: Berry & Burnell, Handbook of Astronomical Image Processing (Willman-Bell, 2011); sec. 10, and Henden & Kaitchuck, Astronomical Photometry (Willman-Bell, 1980); ch. 1]