The GRAVITY spectrometers: design report of the optomechanics and active cryogenic mechanisms

The work package of the University of Cologne within the GRAVITY consortium included the development and manufacturing of two spectrometers for the beam combiner instrument. Both spectrometers are optimized for different tasks. The science spectrometer provides 3 different spectral resolutions. In the highest resolution the length of the spectral lines is close to the borders of the imaging area of the detector. Also the integration time of these high resolution images is relative long. Therefor the optical pathes have to be controlled by the feedback of a faster spectrometer. The fringe tracking spectrometer has only one low resolution to allow much shorter integration times. This spectrometer provides a feedback for the control loops which stabilize the optical pathes of the light from the telescope to the instrument. This is a new key feature of the whole GRAVITY instrument. Based on the optical layout my work was the design of the mechanical structure, mountings, passive and active adjustment mechanisms. This paper gives a short review about the active mechanisms and the compliant lens mounts. They are used similarly in both spectrometers. Due to the observation and analysis of near-infrared light the mechanisms have to run at cryogenic temperatures and in a high vacuum. Except the linear stages, the motorized mechanisms will get used for several times per observation.