Commissioning of the FORS instruments at the ESO VLT

ABSTRACT FORS (FOcal Reducer/low-dispersion Spectrograph) is an all dioptric focal reducer designed for direct imaging, low-dispersion multi-object spectroscopy, imaging polarimetry and spectropolarimetry of faint objects. Two almost identical copies of the instrument (FORS1 and 2) were built by a consortium of three astronomical institutes (Landessternwarte Heidelberg and the University Observatories of Göttingen and München) under contract and in cooperation with ESO. FORS1 was installed in September 1998 and FORS2 in October 1999 at the Cassegrain foci of the ESO VLT unit telescopes nos.1 and 2. FORS1 is in regular operation since April 1999. Regular observations with FORS2 are scheduled to begin in April 2000. There were two commissioning periods foreseen for each of the instruments to test the performance and reliability under various observing conditions. Extensive tests were done on the electro-mechanical functions, image motion due to flexure, optical quality, instrument software, calibration and in particular on the multi-object spectroscopy. Also a detailed characterization of the instrument's properties in the different observing modes has been carried out. In this paper, the procedures of the tests and the results obtained during the commissioning runs are presented in detail and compared with the specifications