The GRAVITY acquisition and guiding system

GRAVITY is a VLTI second generation instrument designed to deliver astrometry at the level of 10 μas. The beam transport to the beam combiner is stabilized by means of a dedicated guiding system whose specifications are mainly driven by the GRAVITY astrometric error budget. In the present design, the beam is monitored using an infrared acquisition camera that implements a mosaic of field, pupil and Shack-Hartmann images for each of the telescopes. Star and background H-band light from the sky can be used to correct the tip-tilt and pupil lateral position, within the GRAVITY specifications, each 10 s. To correct the beam at higher frequencies laser guiding beams are launched in the beam path, on field and pupil planes, and are monitored using position sensor detectors. The detection, in the acquisition camera, of metrology laser light back reflected from the telescopes, is also being investigated as an alternative for the pupil motion control.