The final design of the GRAVITY acquisition camera and associated VLTI beam monitoring strategy

The GRAVITY acquisition camera measurements are part of the overall beam stabilization by measuring each second the tip-tilt and the telescope pupil lateral and longitudinal positions, while monitoring at longer intervals the full telescope pupil, and the VLTI beam higher order aberrations. The infrared acquisition camera implements a mosaic of field, pupil, and Shack Hartman type images for each telescope. Star light is used to correct the tip-tilt while laser beacons placed at the telescope spiders are used to measure the pupil lateral positions. Dedicated optimized algorithms are applied to each image, extracting the beam parameters and storing them on the instrument database. The final design is built into the GRAVITY beam combiner, around a structural plane where the 4 telescope folding optics and field imaging lenses are attached. A fused silica prism assembly, kept around detector temperature, is placed near to the detector implementing the different image modes.