AN OCCULTATION BY SATURNS RINGS ON 1991 OCTOBER 2-3 OBSERVED WITH THEHUBBLE-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 Teles cope (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 inter rupted 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 interv al from UTC 1991 October 2, 19:35 until UTC 1991 October 3, 15:35. Occ ultations by 43 different features throughout the classical rings were unambiguously identified in the light curve, with a second occultatio n by 24 of them occurring due to spacecraft orbital parallax during th is extremely slow event. Occultation times for features currently pres umed circular were measured and employed in a geometrical model for th e rings. This model, relating the observed occultation times to featur e radii and longitudes, is presented here and is used in a least-squar es 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., Icar us, 103, 215 (1993)] yield a ring-pole direction of alpha = 40.5929-de grees +/- 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 .