D. Pascu
U.S. Naval Observatory
A program of astrometric CCD observations of faint satellites of the planets was initiated at the U.S. Naval Observatory (USNO) to complement their photographic program for the bright satellites (Pascu 1979, Pascu et al. 1983). The program emphasizes the faint inner satellites for which the CCD is better adapted. Included in this group are Amalthea (JV), Thebe (JXIV), Helene (SXII), Telesto (SXIII), Calypso (SXIV), Miranda (UV), and Nereid (NII). Almost all of the faint outer satellites are also being observed with a CCD. In addition, an experimental program for high precision observations of the Galilean satellites was begun two years ago in support of the GALILEO Project.
The quantum efficiency and linear response of the CCD detector have been largely responsible for its success in satellite astrometry. However, the small size of the CCD detector makes it more difficult to solve the classical astrometric problems of scale, orientation, and coordinate origin, and special instruments and techniques are necessary to accomplish this.
At present, 25 satellites cannot be observed from the ground at all, or with great difficulty. Of these, about 15 can be detected with the CCDs in the Wide Field Planetary Camera II of the Hubble Space Telescope. The Naval Observatory participates on a team which is analyzing recent HST astrometric observations of the inner Uranian system and is making plans to obtain similar observations of the inner Neptunian system.