As described earlier, the primary effect on optical astronomy comes for cases in which the sky brightness is the limiting factor. For those cases, equation (1) was simplified to: Here we can see that required observing time is directly proportional to our pollution factor p. This equation also shows that when working on faint sources, increase in light pollution is equivalent to reducing the aperture of a telescope by a factor of l//p . For the largest American telescope, the 200-in. telescope at Mt. Palomar, the effect of doubling the sky brightness would be to reduce the aperture of the telescope to 140 in. The threshold values for effects on optical astronomy described above were arrived at in the following fashion: 1. Noticeable effect (P = 1.1 - 1.3) - Whereas we recognized that some members of the astronomical community would find any additional source of diffuse sky brightness unacceptable, we believe p = 1.1 as a result of SPS to be the threshold for a demonstrable impact on optical astronomy. This figure is the usual value quoted for the onset of "light pollution" for the purposes of light abatement ordinances with cities near observatories. The value represents a 10% increase in the night sky brightness and is equal to about 1/2 of the natural variation of the airglow over the 11-year solar cycle. As a further reference, at Kitt Peak National Observatory, the lights of Tucson, Arizona result in p = 1.07 at the zenith. 2. Severe hindrance (p = 1.3 - 2.0) - It should be clear that this level of light pollution will have a demonstrable effect on optical astronomy. The lower range (p = 1.3) means that observations will take 30% longer to reach the same level of statistical accuracy; p = 2.0 corresponds to the doubling of observing time.
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