At the Naval Observatory, the CCD is applied to all satellites fainter than 13th visual. For those satellites near their primary, the 61-inch astrometric reflector at Flagstaff is used, together with a coronagraph (belonging to Lowell Observatory) to reduce diffracted light around the planet. A Texas Instruments 800 X 800 CCD chip, prototype of the Hubble Space Telescope (HST) Wide Field/Planetary Camera (WFPC), is used for all of these observations, usually made in the WFPC wide BVRI filters. Scale/trail reductions are used in most cases. Scale is derived nightly for each filter from a small scale field in the Pleiades or from short exposures of the bright satellites; the precision is about 5 parts in 10,000. Orientation is derived from star trail exposures or from two well-spaced bright satellites. The precision of the mean rotation, derived from several trails, is about +/-0.01 degree. Although a partially transparent filter is used as the coronagraphic stop, the planetary image is not measured (except for Neptune), and the measured coordinates of the faint satellites are referred to the bright ones. This is done in different ways for each system and is explained below. Measurement consists of modeling the satellite images as 2-dimensional Gaussians. In case the satellite is involved with the halo light of the primary, the background is modeled as a planar or quadratic surface.
Satellites of Jupiter
Amalthea (JV) and Thebe (JXIV)
Observations of these two satellites were begun in 1981 (Pascu and Seidelmann 1981) shortly after Thebe's discovery by Synnott (1980). Our technique was to measure Thebe relative to Amalthea. Because Amalthea can only be observed for about 3 hours a night, observations were begun 1.5 hours before Jupiter transit and 1.5 hours prior to elongation, when Thebe would be visible for at least 3 hours. While such configurations are few during Jupiter's apparition, they will last for several days. Amalthea's 12 hour period will place it in the same position on following nights, while Thebe's 16 hour period will place it on opposite sides of it's orbit -- alternating from night to night.
The latest observations, made at the opposition of 1992 (Pascu et al. 1992b), gave systematic (O-C)'s larger than 2 arcsec. and indicated that Thebe was advanced on its orbit. Jacobson (1994a) fit a Struve theory to all the recent observations, reducing the residuals of our 1992 observations to +/-0.067 arcsec. The post-fit residuals for small separations, +/-0.037 arcsec, agreed well with the expected external precision for those separations, but the residuals for larger separations were 50 systematic scale-like signal (Pascu et al. 1994b).
JVI - JXII
Observations of the outer satelliets were begun in 1993 by Rohde (1994) and DeYoung with the USNO 24-inch in Washington. A Photometrics 1024 X 1024 CCD is used, which has an 11 arcmin field. Reference stars from the HST Guide Star Catalog are used to reduce the frames, giving right ascensions and declinations for the satellites. Positions relative to Jupiter are computed using an ephemeris of the planet. The external precision of these observations is about +/-0.5 arcsec. This work shows the potential for faint satellite astrometry with a small instrument at a bright sky site.
Galilean Satellites (JI-JIV)
In an attempt to provide very high precision orbits (10 km) of the Galileans for their encounter with the crippled Galileo spacecraft, the Jet Propulsion Laboratory and the Naval Observatory (Monet 1993) have begun a program aimed at milliarcsecond (mas) astrometry of the Galilean satellites. The 61-inch astrometric reflector at Flagstaff is used together with a Techtronics 2048 X 2048 CCD. Many frames a night are taken of the satellites through an attenuating filter (to increase the exposure time). In the astrometric analysis, nightly normal points are formed and nightly stochastic corrections are determined to the scale and orientation of the CCD frames. The objective is to obtain nightly normal points with an accuracy of about +/-10 mas, then to reduce the error further by making correction for phase and surface albedo variations. To date, the experiment is partially successful; while the normal points did have a precision of +/-10mas, the phase and albedo variation corrections did not produce the expected improvement (Null 1994).
Satellites of Saturn
Janus (SX), Epimetheus (SXI), Helene (SXII), Telesto (SXIII), and Calypso (SXIV) These five faint satellites were discovered or recovered in ground- based observations at the 1980 ring-plane crossing. Observations of all five were made at the Naval Observatory in that year, and observation of Helene, Telesto and Calypso were continued, but we have been unable to detect Janus and Epimetheus with our equipment since then. For these observations, the scale and orientation can be easily determined by the methods outlined above, but the problem of coordinate origin is not simple due to the large magnitude difference between these three faint satellites and the bright ones. In an exposure long enough to detect the faint satellites, the bright ones are overexposed and saturated.
We use two methods to reference the faint satellites to the bright ones. The first method involves making the observations when the bright satellites are behind the partially transparent coronagraphic stop. This takes a lot of planning and restricts the orbital positions of the faint satellites. It works best when the rings are open, and not at all when the observations are made within two years of ring-plane passage because the bright satellites pass too close to the planet and rings to be measured. The more general method requires that short exposures, for the bright satellites, be interspersed among the longer ones for the faint satellites. These are used to plot the X(t) and Y(t) motions of the bright satellites on the CCD chip relative to background stars. Quadratics are fit to the X(t) and Y(t) functions, enabling one to obtain the X, Y coordinates of the bright satellites on the CCD chip at the instant of exposure for the faint satellites. This yields, finally, positions of the faint satellites relative to the bright ones.
Results to date (Rohde and Pascu 1993, 1994) show that Telesto's ephemeris is still quite serviceable, with an observational error of +/-0.2 arcsec. Calypso, however, shows small systematic radial deviations from its orbit in the 1993 observations, but not in the 1992 observations. Helene, the brightest of the Lagrange librators, had the largest errors in its ephemeris. The 1992 observations show Helene advanced on its orbit by 8 degrees while the 1993 observations show it lagging behind by 10! This indicates that the librational term in the longitude needs revision. The libration, however, is very sensitive to the mass of Dione, and a correction to the mass of Dione was found by Jacobson (1994b), who recently fit an integration to these observations. The external error for these observations was also about +/-0.2 arcsec.
Phoebe (SIX)
Observation of this outer satellite of Saturn was begun in 1992 by Rohde (1994) with the USNO 24-inch in Washington. The techniques used are identical to those used in the observations of the outer satellites of Jupiter. As for the outer Jovian satellites, it is the HST Guide Star Catalog which has made it possible to apply CCDs to faint outer satellite systems. The external precision of these observations is about +/-0.5 arcsec.
Satellites of Uranus
Miranda (UV)
CCD observations of Miranda were made from 1981 to 1988 to complement our photographic observations of the brighter satellites. The observations of 1981 - 1985 were reduced to assist the Voyager reconnaisance of Miranda (Pascu et al. 1987). The problems of scale, orientation and coordinate origin were solved simultaneously by using the four bright satellites as reference objects. Predicted positions for the four bright satellites were used to make a plate-solution for each CCD frame. A comparison of the observations to an integration yielded an external error of +/-0.10 arcsec.
UVI - UXV
These satellites have not been observed since their discovery with Voyager in 1985/6. Only the brightest of them can be detected from the ground, and then with great difficulty. However, several of these satellites can be detected with the Hubble Space Telescope, and Naval Observatory astronomers have joined with those from the Space Telescope Science Institute and the University of Maryland in a project to detect and study these satellites with HST (P.K. Seidelmann, P.I.). Observations made in August of 1994 were successful in detecting 5 or 6 of the brightest of these satellites and the rings. Astrometric, dynamical and photometric studies are in progress (Zellner et al. 1994).
Satellites of Neptune
Nereid (NII)
CCD observations of Nereid were made from 1981 to 1988 to complement our photographic observations of Triton. Scale was provided by the scale region in the Pleiades and orientation from star trails. The filtered image of Neptune or unfiltered image of Triton were measured and used as the coordinate origin. Several of these observations were used by JPL to assist at the Voyager reconnaisance of Neptune. The external precision of these observations was about +/-0.25 arcsec.
NIII - NVIII
As in the case of the inner satellites system of Uranus, this inner system of Neptune has not been observed since its discovery by Voyager in 1989 except for a ground-based detection of Proteus by Colas and Buil (1992). Ground-based detections, however, require heroic efforts and involve methods which are not conducive to good astrometry. With the success of the HST detection of the inner Uranian system, the Naval Observatory has also joined in a proposal with astronomers from the Space Telescope Science Institute and the University of Maryland to obtain observations of these faint inner satellites for the purpose of astrometry and photometry, in preparation for a spectroscopic follow-up the next year.