Design and prototype performance of an innovative cryogenic tip-tilt mirror

Cryogenic mechanisms are needed for the alignment plan of MATISSE, a mid-infrared spectro-interferometer for the European Southern Observatory Very Large Telescope Interferometer (ESO VLTI) that combines up to four Unit Telescopes or Auxiliary Telescopes. Telescope beams are split into 16 beams that need to be aligned on the detector and corrected for OPD (Optical path difference) in order to create an interference pattern. Alignment accuracy and stability specifications are of the order of nanometers and arcsec's. These specifications cannot be met by warm alignment nor by manufacturing tolerances, therefore 16 motorized tip-tilt units are needed that operate in a vacuum cryogenic environment. Key aspects of the mechanisms are that the optical element and mechanism are combined in a compact single component, driven by self braking piezo actuators in order to hold position without power. The design, realization and test results of these mechanisms are presented.