Quasometry, Its Use and Purpose
  Quasometry is precision measurement of celestial positions and apparent
motion of very distant extragalactic objects, such as quasars, galactic nuclei,
and QSOs. We use this term to identify a specific area of research, the
methodology of which differs from that of general astrometry. The main purpose
of quasometry is to link the sub-milliarcsecond radio frame (ICRF) with the
existing and emerging optical reference frames of similar accuracy, constructed
by astrometric satellites. Some of the main difficulties in achieving this goal
are discussed, e.g., the extended structures of quasar hosts, apparent motion
on the sky, optical variability, galactic companions, faintness. Besides the
strategic purpose, quasometry is undoubtedly useful for global astrometric
surveys, as it helps to verify or even correct the resulting reference frames.
There are two options of using measurements of distant quasars in a global
astrometric solution: 1) hard constraints embedded in the fabric of
observational equations; 2) {\it a posteriori} fitting of zonal errors. The
relative benefits and shortcoming of the two options are reviewed. A relatively
small set of about 200 carefully selected reference quasars can go a long way
in improving the astrometric value of a space mission, if they are sufficiently
bright, stable, fairly uniformly distributed on the sky, and are defining
sources in the ICRF. We present an ongoing program at the USNO to construct a
quality set of optical quasars with the required properties and to enhance the
ICRF with new sources in the areas where known, well-observed quasars are
scarce.
