SPECIFIC COMMENTS The effects of increased sky brightness on astronomical measurements have been well treated elsewhere in this workshop. Effects may slightly vary depending on the type of detector and telescope. For example, photographic film is much more sensitive to increasing sky background than is a linear detector with a large storage capacity. But it is clear that we will have to rely on photographic film as a simple yet effective detector for large formatimaging applications for some time to come. For a linear, photon-counting detector the limitation is the sky background. Most of the significant work being done in optical astronomy now involves observing objects at levels between 1/10 and 1/100 of the sky background. At these levels the effective observing time needed to achieve a given accuracy with a given telescope scales linearly with the increase of sky background. However, beyond an exposure time of several minutes, the stability of instrumentation and observing conditions becomes a negative factor. It is impossible to express this rigorously, but the effective limit to exposure time is about one hour. Some of the assumptions used in preparing the Briefing Document for the workshop do not appear to be valid, specifically: 1. The light scattered back into the atmosphere should not be ignored in computing the increase of sky brightness. This will be variable in time and place. In winter with a snow blanket the present measured sky brightness at places such as Lowell Observatory increases markedly. For Mauna Kea, which is surrounded by a high albedo ocean, the backscattering must be significant. It will be difficult to estimate, but some attempt should be made. 2. The assumption of 4% reflectivity for the concentrator configuration is not valid. The configuration will act as a corner cube (within certain limits) and will effectively reflect from the whole area. From half the area the angle of reflection off the sapphire cover will be 30°, which should increase the reflectivity. I would expect the total reflectivity to be closer to 8%.
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