DDA2001, April2001
Session 1. Very Small Things
Monday, 8:40-10:10am, (Coffee Break 10:10-10:30am), 10:30am-12:00noon

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[1.03] Adaptive Optics Observations of Saturn's Inner Moons

B. R. Scharringhausen, P. D. Nicholson, T. L. Hayward, B. R. Brandl (Cornell University), E. E. Bloemhof (Caltech), R. G. Dekany, M. Troy (JPL)

Saturn's moons Pandora and Prometheus are challenging targets for observation. They are small and therefore faint, and their proximity to the bright A ring means that they are usually lost in the scattered light from the rings. A 19 degree lag in Prometheus' orbit was discovered during the ring-plane crossing of 1995, and subsequent observations have uncovered a lag in the orbit of Pandora, as well, generating interest in the further observations of these objects.

For a small range of ring-opening angles, the outer edge of the rings is covered by the disk of the planet, but the satellites just beyond the rings are not. The satellites can be imaged as they move through the "gap" in the rings near Saturn's pole, where the scattered light from the rings is greatly reduced. In 1990, Nicholson et al. (1992 Icarus 100, 464.) observed Janus and Epimetheus in this geometry. It was possible to observe Pandora and Prometheus in a similar manner when the ring opening angle was between -23.40 to -23.89 degrees in 2000. As they traverse the "gap," the moons are at most 0.35 arcseconds from the limb of the planet. Adaptive optics allowed us to make this observation from the ground.

On November 9 and 13, 2000, we imaged Prometheus, Janus, and Epimetheus using the Adaptive Optics system at the 5-m Hale Telescope at Palomar Observatory. The K filter (2.0-2.4 microns) is in a methane absorption band, making the disk of the planet very dark. Prometheus' observed position was within 0.25 degrees of an extrapolation of the increasing orbital lag measured by French et al. (1999 DDA #31, #90.01.) from HST images, i.e. 21.9 degrees. Janus and Epimetheus were also observed at their predicted positions.

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