SIM PlanetQuest is designed as a space-based 9-m baseline optical Michelson interferometer operating in the visible waveband. This mission will open up many areas of astrophysics, via astrometry with unprecedented accuracy. Over a narrow field of view (1°), SIM aims to achieve an accuracy of 1 µas in a single measurement. In this mode, SIM will search for planetary companions to nearby stars, by detecting the astrometric 'wobble' relative to nearby reference stars.

For targets separated by larger angles SIM has the goal to reach 4 µas precision in absolute position measurements at the end of the 5-year mission. These wide-angle measurements will be made relative to an astrometric grid of reference stars covering the entire sky. The astrometric grid is constructed from repeated relative measurements of the separation of stars within overlapping "tiles." Each tile has a diameter of 15 degrees and represents the area of the sky accessible to SIM by repointing the optics without changing the orientation of the spacecraft.

SIM Instrument and Mission Parameters
Baseline	9 m
Wavelength range	0.4 - 0.9µm;
Telescope Aperture	0.3 m diameter
Astrometric Field of Regard	15° diameter
Astrometric Narrow Angle Field of Regard	1° diameter (2° oval)
Detector	Si CCD
Orbit	Earth-trailing solar orbit
Science Mission Duration	5 years
Wide Angle Astrometry	4 µas mission accuracy (goal)
Narrow Angle Astrometry	1 µas single measurement accuracy (goal)
Limiting Magnitude	20 mag (goal)