It is exactly these considerations that led to Gallagher and Faber’s equation (see their invited paper) that gives the amount of time necessary to reach a given signal-to-noise ratio (fractional error). In this equation, g is the ratio of the brightness of the object to the brightness of t/ie natural sky, p is the ratio of the actual sky brightness to the natural sky brightness (p always - 1), R is the desired signal-to-noise ratio, A x N is the brightness of natural sky in photons/sec/cm , Q is the detector efficiency and o is the collecting area of the telescope in cm2. For the case of faint objects: which means that required observing time is proportional to the brightness of the sky and inversely proportional to the collecting area of the telescope. It is easy, then, to understand why astronomers need dark skies and large telescopes to observe faint objects. Astronomical Accommodation to Existing Sources of Sky Brightness Natural and man-made sources of sky brightness already have a significant impact on the conduct of optical astronomy. The largest natural contribution to the diffuse night sky brightness comes from the moon. The moon has such a profound effect on the sky that observatories routinely divide their observing schedule into "dark" (no moon) and "bright" (moon up) time. Generally, dark time is in much greater demand than bright time on large telescopes, as seen in the distribution of visitor proposals at Kitt Peak National Observatory (KPNO) for dark time and bright time for February 1, 1979 to July 31, 1979 (Table 1).
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