Once the optical astronomy working group convened, it became increasingly clear that optical aeronomy, represented by K. Clark of the University of Washington, deserved attention beyond that which could be provided in the optical astronomy report. As such, aeronomy has been separated out as a third topic of the workshop. This separation is quite appropriate because a large fraction of the natural background sky brightness comes from aurorae and airglow phenomena studied by aeronomers. Since the effects studied produce relatively diffuse light emission which is especially difficult to distinguish from other increases in sky brightness, the thresholds for impacts on aeronomy are lower than they would be for optical astronomy. BASIC CONCLUSIONS OF THE WORKSHOP While the effects of the SPS on astronomical research are quite diverse, particularly as they apply to the radio and optical regimes of the electromagnetic spectrum, there are two important effects that have a common origin that will affect both areas of research. The geostationary character of the satellite orbits means that the satellites will occupy the same portion of the sky at all times. Therefore, a fixed region of the sky will be unusable for astronomical research. How large this region is depends on the design of the satellites, the particular observation being made, and the kind of instrumentation being used. A further effect is that placing the source of electromagnetic interference and light pollution relatively high in the sky will essentially eliminate terrain shielding as a strategy to combat pollution and interference effects. The primary effect on optical astronomy was attributed to increased sky brightness. Any increase in the brightness of the sky results in a proportional reduction in the effective aperture of a telescope when it is being used on faint sources. The predicted increases of sky brightness suggest that at a minimum any western hemisphere observatory will be prevented from effectively observing faint sources in a region that covers 10° in declination and 70° in hour angle surrounding the line of satellites. There will also be, again as a minimum, a noticeable effect on observation over a region that covers more than 60° in declination and 90° in hour angle, approximately half
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