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 10μas accuracy at K-band. We present the system design of the science and fringe tracking spectrometers of GRAVITY: The fringe tracking spectrometer is optimised for highest sensitivity, providing a fixed spectral resolution. The science spectrometer provides 3 different low - medium spectral resolutions. Both spectrometers provide detector focus stages and deployable Wollaston prisms. The two spectrometers also feed the beams of the metrology laser system of GRAVITY backwards into the integrated optics beam-combiner, propagating back to the M2 mirrors of the 4 telescopes.