IRTF Observations of the Occultation of 28 Sgr by Saturn

We obtained an infrared-imaging time series (λ = 3.255 μm, Δ t = 0.25 sec) of the 1989 July 3 occultation of 28 Sgr by Saturn and its rings using the NASA Infrared Telescope Facility (Mauna Kea, HI) and the University of Rochester 62 × 58-pixel InSb array camera. The radial extent of the beam in the ring plane was ∼20 km, and the rings were sampled approximately every 5 km. The images show stellar signal throughout the ring event, including the densest parts of the B ring, although some signal may be due to indirect light diffracted into the beam (P. D. Nicholson, O. Perkovic, K. Matthews, and R. G. French 1991, Bull. Am. Astron. Soc. 23, 1178). The time associated with each pixel readout is accurate to within 60 μsec of UTC. We present a table of interpolated ring event times. Although qualitative ring structure at scales of 10-100 km is consistent with the Voyager stellar and radio occultation profiles, our profile varies systematically with respect to the Voyager data over large radius scales. The most likely explanation for this is indirect stellar signal diffracted through the rings; to date there is no quantitative model of this effect, and the ring masses derived here have not been corrected for it. The projected stellar diameter of ∼20 km put most density- and bending-wave trains below our resolution; fitting models to these waves for surface mass density was therefore not attempted. The Voyager Photopolarimeter Subsystem occultation data do not suffer such smearing, so the mass extinction coefficient determined by L. W. Esposito, M. O'Callaghan, and R. A. West (1983, Icarus 56, 439-452) was used to estimate the mass of the rings, based on the 1989 July 3 data. The masses derived in this fashion are (2.42 ± 0.93) × 10 ²² (total), (0.097 ± 0.037) × 10 ²² g (C ring), (1.84 ± 0.71) × 10 ²² g (B ring), and (0.49 ± 0.19) × 10 ²² g (Cassini division and A ring). We present masses and mean optical depths of individual ring sections.