The SIM Lite telescope functions through optical interferometry. SIM was to be composed of one science interferometer (50 cm collectors, 6 m separation a guide interferometer (30 cm collectors, 4.2 m baseline), and a guide telescope (30 cm aperture). The sophisticated guide telescope stabilizes instrument pointing in the third dimension. The spacecraft's operational limiting magnitude would have gone down to 20 at 20 millionths of an arcsecond (μas) and its planet-finding, astrometric accuracy of 1.12 µas is for single measurements. The accuracy of its global, all-sky astrometric grid would have been 4 µas.