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D. J. Tholen (Inst. for Astronomy, Univ. of Hawaii)
Previous observations of Charon's orbit acquired with the Hubble Space Telescope revealed a small but significant nonzero eccentricity (Tholen and Buie, Icarus 125, 245-260, 1997). Part of the apparent eccentricity is probably due to offsets between the center of light and the center of mass caused by variegation, though the best available models were unable to account for all of the measured eccentricity. In an attempt to refine the orbit determination for Charon, new observations were made using the Institute for Astronomy's "Hokupa'a" adaptive optics camera attached to the Gemini North telescope on Mauna Kea. Images were acquired through the H filter on eight separate nights spanning a full year (2001 April 19 and 28 UT; 2001 June 23, 25, and 26 UT; and 2002 April 23, 24, and 26 UT). These eight epochs provide a reasonably uniform distribution in orbital longitude. The image quality varies, with the best images achieving a FWHM of 0.09 arcsec, which is marginally less than Pluto's 0.11 arcsec disk diameter. Calibration of image scale and position angle orientation were accomplished by watching asteroids of known motion move past field stars of suitable brightness. The approximate image scale is 20 milliarcsec per pixel, so the point spread function is well sampled. Reductions are underway, and the latest results will be presented at the meeting.
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Bulletin of the American Astronomical Society, 34, #3< br> © 2002. The American Astronomical Soceity.