This paper summarizes the results of a two-year survey of potential darksky observing sites in California south of the latitude of San Francisco. This survey was undertaken in 1965 to locate a new site for friture large optical telescopes since the growth of lights and smog near Mount Hamilton are increasingly limiting the usefulness of that site for observations of very faint objects and for photometry.The best observing conditions appear to occur along the coast, very close to the ocean where the cold ocean current holds down the height of the inversion layer, and where there exists a laminar airflow off the ocean, minimizing the optical turbulence and placing the site upwind from sources of smog.
Measures at San Benito Mountain during the period from 1976-87 show that the zenith sky brightness in the V and B photometric bands is well correlated with the solar 10.7-cm radio flux, and thus with the intensity of the solar EUV radiation in the 100 A-625 A range. A change of at least AV = 1.0, AB = 0.8 mag may occur as the EUV intensity varies with solar activity. In addition, the brightness of the zenith sky is found to decrease exponentially by about 0.4 mag in both V and B during at least the first half of the night.These results indicate that:(1) The level of solar EUV flux and the time of night must be taken into account when assessing the quality of a dark-sky site.(2) In order to obtain the best signal-to-noise ratio for limiting observations in the V and B filter bands, it is of importance to observe at times of minimum solar EUV radiation and in the latter part of the night.(3) Nighttime sky brightness may be predictable with reasonable accuracy from the intensity of the 10.7-cm radio flux observed during the preceding day.
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