SIM will yield 4 microarcsecond (mas) absolute
position and parallax measurements. In order to accomplish this, SIM will
define a new astrometric reference frame, using a grid of approximately 1300
stars with positions accurate to 4 mas. Unlike most survey instruments,
SIM will maintain its astrometric accuracy down to the faintest magnitudes.
This ability opens up the opportunity for astrometry of active galactic
nuclei to better than 10 mas at magnitudes fainter than about 16.
A precision frame tie between optical and radio frames provided through
astrometry of optical (radio-loud) quasars by SIM. The current radio-based
(quasar) reference frame is the International Celestial Reference Frame,
which is tied to optical observations through observations of radio-bright
stars with Hipparcos. Chromospherically active stars are very interesting
scientifically, but they make poor reference objects because of their extended
sub-structure (see Stellar Astrophysics). By using quasars, the SIM frame tie will eliminate the need to rely on stars.
SIM Science Investigations