AN OCCULTATION BY SATURNS RINGS ON 1991 OCTOBER 2-3 OBSERVED WITH THE HUBBLE-SPACE-TELESCOPE

An occultation of the star GSC 6323-01396 (V = 11.9) by Saturn's rings was observed with the High-Speed Photometer on the Hubble Space Telescope (HST) on 1991 October 2-3. This occultation occurred when Saturn was near a stationary point, so the apparent motion of Saturn relative to the star was dominated by the HST orbital motion (8 km s-1). Data were recorded simultaneously at effective wavelengths of 3200 and 7500 angstrom, with an integration time of 0.15 s. Observations were interrupted by passages of the spacecraft behind the Earth and through the South Atlantic Anomaly. Fifteen segments of occultation data, totaling 6.8 h, were recorded in 13 successive orbits during the 20.0 h interval from UTC 1991 October 2, 19:35 until UTC 1991 October 3, 15:35. Occultations by 43 different features throughout the classical rings were unambiguously identified in the light curve, with a second occultation by 24 of them occurring due to spacecraft orbital parallax during this extremely slow event. Occultation times for features currently presumed circular were measured and employed in a geometrical model for the rings. This model, relating the observed occultation times to feature radii and longitudes, is presented here and is used in a least-squares fit for the pole direction and radius scale of Saturn's ring system. Combined fits with the HST occultation times and 28 Sgr occultation times [French et al., Icarus, 103, 163 (1993) and Hubbard et al., Icarus, 103, 215 (1993)] yield a ring-pole direction of alpha = 40.5929-degrees +/- 0.0151 and delta = 83.5348-degrees +/- 0.0053-degrees (J2000.0, at the Voyager 1 epoch of UTC 1980 Nov. 12 23:46:32). This result, independent of Voyager data and its associated trajectory errors, is compared with other recent determinations of the pole and radius scale.