Testing FORS: the first focal reducer for the ESO VLT

FORS1 (FOcal Reducer/low-dispersion Spectrograph) is an all dioptric focal reducer designed for direct imaging, low- dispersion multi-object spectroscopy, imaging polarimetry and spectro-polarimetry of faint objects. Two identical copies of the instrument (FORS 1 and 2) are being built by a consortium of three astronomical institutes (Landessternwarte Heidelberg and the University Observatories of Gottingen and Munich) under contract and in cooperation with ESO. FORS 1 and 2 will be installed, respectively, in 1998 and 2000 at the Cassegrain foci of the ESO VLT unit telescopes nos. 1 and 2. For the tests of FORS in Europe, a telescope and star simulator was built, which allows to incline and rotate the whole instrument and to simulate stars in the field of view at various seeing conditions. FORS 1 was integrated at the telescope simulator and saw its 'first light' in the integration facility in November 1996. Since then the electro-mechanical functions, the image motion due to flexure, the calibration units, the optical performance and the instrument software were tested and optimized. This paper presents a summary of the procedure and the results of the tests.