Photon detectors are affected differently, as it is possible to compensate for increased sky brightness by integrating for longer times. The time 9 to reach a given signal-to-noise ratio R over a region of area A arcsec with a telescope of collecting area 0 equipped with a detector of efficiency Q is Alternatively, the increase in sky brightness decreases the intensity level detectable with a given telescope and fixed integration time by a factor of l//p~ . The variation in ts and the magnitude loss factor for photon counting detectors are also given in Table 1, and we see that for an ideal photon detector the SPS background outside of a contaminated zone of 20°x75° is marginally tolerable and is objectionable over 40°x80°. Within this smaller contaminated zone, telescope performance is reduced below an acceptable level. How will these contaminated areas affect astronomy? Note first that the parallax between will be sufficiently large in declination (10°) such that the contaminated zones for photon detectors overlap in only a small area. Thus, between these two observatory sites, and assuming that by 2000 use of photon detectors will predominate, little of the sky would be irrevocably lost for faint object work. If we take only KPNO and assume that sources will be observed within ±2 hours (±30°) of the meridian, then in principle about 1/6 of the sky will be lost, and studies of objects distributed at random would not be seriously affected. The brightest examples of various astrophysical systems are, however, not distributed at random, and we might well lose access to critical individual objects. For example, the contaminated band as seen from KPNO lies within declinations of +4° to -16°, which includes a selection of very interesting objects such as the bright globular cluster M3, the Orion Nebula, some important galaxies, and several interesting but faint quasars in the equatorial zone. The extra light contributed by the SPS would be detrimental to frontier optical astronomy.
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