Design and construction of the Max-Planck-Institut fuer Extraterrestrische Physik beam combiner simulator COSI

We present the layout and construction of main parts of the imaging beam combiner simulator experiment COSI, which has been set up at the Max Planck Institut fuer extraterrestrische Physik. The purpose of COSI is to simulate the imaging beam combiner at the coherent focus of the ESO VLTI. Main objectives of the experiment are to explore near-infrared array detector properties and to generate data for testing (developing) image reconstruction algorithms. COSI consists of a 1-m telescope and a near-infrared continuum light source to simulate the radiation from astronomical objects. Close to the focal plane, the beam is split into two foci. The beam is relayed between the two parts of the telescope by a roof mirror. Between the telescope and the retroreflector in the quasi-parallel beam, a pupil mask allows us to define a number of beams, thus simulating various telescope configurations with a beam compression factor of 100. The light from the pin-hole source is transmitted through the telescope system to the near-infrared camera placed in the receiving focus. The wavefront in the beams can be influenced by seeing simulators. Presently we use hot air devices, but for the final configuration we aim at deformable mirrors to have detailed control over the wavefront. Experimental results on image recovery achieved up to now are reported elsewhere in this conference.