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 the infrared K-band.
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.
Optically two individual components, both spectrometers are two separate units with their own housing and interfaces
inside the vacuum vessel of GRAVITY. The general design of the spectrometers, however, is similar. The optical layout
is separated into beam collimator (with integrated optics and metrology laser injection) and camera system (with
detector, dispersive element, & Wollaston filter wheel). Mechanically, this transfers to two regions which are separated
by a solid baffle wall incorporating the blocking filter for the metrology Laser wavelength. The optical subunits are
mounted in individual rigid tubes which pay respect to the individual shape, size and thermal expansion of the lenses.
For a minimized thermal background, the spectrometers are actively cooled down to an operating temperature of 80K in
the ambient temperature environment of the GRAVITY vacuum dewar. The integrated optics beam combiner and the
metrology laser injection, which are operated at 200/240K, are mounted thermally isolated to the cold housing of the
The optical design has shown that the alignment of the detector is crucial to the performance of the spectrometers.
Therefore, in addition to four wheel mechanisms, six cryogenic positioning mechanisms are included in the mechanical
design of the detector mount.