Pointing and tracking
The UTs have a pointing accuracy of 3 arcsec RMS. The expected tracking accuracy under nominal wind load is 0.1 arcsec RMS over 30 minutes when field stabilization is active. The UTs also have the capability of tracking targets with additional velocities (e.g. Solar System targets) under full active optics control. Proposers who need this capability should specify the additional velocities in RA and Dec for their targets.
Active optics guiding
For all observations a guide star is used for acquisition, active optics, and field stabilization. The typical guide star R magnitude ranges from 11 to 14 (in optimal conditions). Observations for which no suitable guide star exists cannot be carried out.
Adaptive optics guiding (VLTI only)
The Coudé foci of the UTs are equipped with MACAO (Multi Application Curvature Adaptive Optics) units, which can be used with natural guide stars with V magnitude in the range of 1 to 17, seeing better than 1.5 arcsec and coherence time smaller than 2 ms. The distance of the natural guide star from the science target must be less than 57.5 arcsec. For observations using the fringe tracker FINITO, the magnitude of the guide star must be brighter than 15 and within 13 arcsec from the science target.