Astrometric catalog around ultracool dwarfs

The third component of unknown origin has an amplitude of 0.03-0.14mas and is independent of the observing conditions. We find that a consecutive sequence of 32 images of a well-exposed star over 40min at 0.6arcsec seeing results in a median r.m.s. of the epoch residuals of 0.126mas. Overall, the epoch residuals are distributed according to a normal law with a {chi}^2^ value near unity.

We compiled a catalogue of 12000 stars with I-band magnitudes of 16-22 located in 20 fields, each covering 2x2'. It contains I-band magnitudes, ICRF positions with 40-70mas precision, and relative proper motions and absolute trigonometric parallaxes with a precision of 0.1mas/yr and 0.1mas at the bright end, respectively. This work shows that an astrometric accuracy of ~100 micro-arcseconds over two years can be achieved with a large optical telescope in a survey covering several targets and varying observing conditions.