Astronomical operations differ markedly from the proposed DOD experiments in the need for stellar guidance input. It is worth noting that although the ATD/NTOT’s collecting area is several times larger than the HST ’s, it has only about 1% of the available guidance field of view. Consequently, the ATD/NTOT must be able to produce a fine-error signal using very faint stars. The fine-guidance system as designed requires one guide star for controlling pitch and yaw and a second for stabilizing roll. The guidance analysis in the Lockheed/Itek briefing concluded that at 19th magnitude there is sufficient guide star availability and that the ATD/NTOT will achieve sufficient signal-to-noise to determine the centroid of a star’s image to 0.003 arc sec, assuming a detector temperature of 260 K with a 10-Hz sampling rate. The reliability of the estimates of signal-to-noise for centroiding was not clear to the task group. More importantly, however, guide stars will be significantly less abundant than assumed, especially at higher galactic latitudes.

The source of the average stellar density data presented in the Lockheed/Itek briefing is not known to the task group, but the briefing assumed an average guide-star density of about 25 per square milliradian at a limiting magnitude of 19. The task group’s calculations, based on the Bahcall-Soneira model (Figure 6.1), show that for latitudes greater than 40 degrees, the density falls below this number, even when looking toward the galactic center. The situation is considerably worse when looking away from the galactic center, with densities almost five times lower. The stellar density remains low over the entire hemisphere facing away from the center of our galaxy and rises by less than 50% in going from longitudes of 180 degrees to 90 degrees. Since the original estimate assumed success in finding guide stars only 80% of the time, it follows from Figure 6.1 that the success rate will be closer to 15% over most of the galaxy.

FIGURE 6.1 The number density of stars brighter than 19th magnitude, in the Bahcall-Soneira model of the galaxy, is shown as a function of galactic latitude for three different galactic longitudes.

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