\documentstyle{article}
 
\title{LONG-FOCUS CCD ASTROMETRY OF PLANETARY SATELLITES}
\author{Dan Pascu \\U.S. Naval Observatory}
\date{} 
\begin{document}
\maketitle 
Twenty-five years ago, the driving motivation for 
satellite astrometry was the need to improve the
obsolete satellite theories by the almanac-producing 
countries. Photographic tangent plane observations
were more than adequate for that task.
 
With the advent of spacecraft reconnaissance of the
major planets and their satellites, there was a need
for more accurate ephemerides.  In addition, many new
satellites discovered in these encounters were
impossible to detect from the ground.  These new
challenges were met with the application of new
techniques, instruments and detectors.
 
The principal advance in astrometric observation in the
past 15 years was the introduction of CCD detectors.  
The quantum efficiency and linear response of the CCD
detector have been largely responsible for its success 
for both tangent plane astrometry and for the 
observation of the mutual phenomena of the bright
moons of Jupiter and Saturn.  However, caution should
be used when applying CCDs to large scale systems
because the small size and complexity of the CCD chip 
make it difficult to calibrate.  This caveat not
withstanding, it is now possible to obtain relative
positions of the satellites with a precision of 5-20
milliarcsec.
 
Of the 25 newly discovered satellites which cannot be
observed from the ground, at least 15 of them can be
observed with the Hubble Space Telescope (HST).  Images
of the eight brightest of the inner satellites of
Uranus have already been detected and measured on 
recent WFPC2 frames.  The astrometric precision of 
these observations is also expected to be in the 5-20
milliarcsecond range.

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