The GRAVITY spectrometers - system design

Operating on 6 interferometric baselines, i.e. using all 4 UTs, the 2nd generation VLTI instrument GRAVITY will deliver narrow angle astrometry with 10 mu as accuracy at the infrared K-band, and will be able to detect the positional shift of the photocenter of a flare at the Galactic Center within its orbital timescale of similar to 20min, using the flares as dynamical probes of the gravitational field around SgrA*. Within the international GRAVITY consortium, the Cologne institute is responsible for the development and construction of the two spectrometers: one for the science object, and one for the fringe tracking object. Both spectrometers are operated at 85 K within a 200 K surrounding in the GRAVITY cryostat. In this contribution we present the current spectrometer system design: The two GRAVITY integrated-optics beam combiners feed each spectrometer with 24 input channels. The science spectrometer fulfills a wide range of scientific requirements with three user selectable spectral resolutions (R=22, R=500, R=4500), while the fringe tracking spectrometer is optimized for high optical throughput and provides a single low spectral resolution of R=22. Both spectrometers feature an optional Wollaston prism for polarimetric studies. In addition to the grism wheel and a mechanism providing the Wollaston deployment, the electro-mechanical design includes focus stages for the science (HAWAII2RG) and fringe tracking (NIR HgCdTe avalanche photodiode arrays) detectors. The two spectrometers also feed the beams of the metrology laser system of GRAVITY backwards into the integrated optics beam-combiner, and special high-OD blocking filters shield the detectors from the metrology light.