Cold optics of MIDI: the mid-infrared interferometric instrument for the VLTI

Abstract

ESO's new Very Large Telescope will consist of four 8.2 m telescopes and three moveable 1.8 m telescopes. Light from these can be combined in the Very Large Telescope Interferometer (VLTI) providing milli-arcsecond resolution with high sensitivity. The VLTI will first operate in the infrared and will produce first fringes in 2001. MIDI is the VLTI instrument for interferometry in the mid-infrared (10 - 20 microns) and is under development by a German-Dutch- French consortium. The initial aim of MIDI is to combine the beams of two telescopes in the 10 micron `N-band' and to achieve spatial resolutions of 20 milli-arcseconds at a spectral resolution of 200 - 300. Modulation of the optical path difference can be done using piezo-driven mirrors at room temperature, but beam combination and detection of the interferometric signal has to be done at cryogenic temperatures due to the `thermal' wavelength domain. The MIDI cold bench is therefore mounted inside a cryostat, cooled by means of a closed cycle cooler to about 40 K for the cold optics and 8 K for the detector. This poster describes the design and implementation of the MIDI cold bench.