ARAL: a facility to fuel VLTI instruments with photons

The ARAL system of the VLTI is a multipurpose facility that helps to have the interferometric instruments ready for night observations. It consists of an artificial source (allowing a Mach-Zehnder mode of the interferometric instruments for autotest), an alignment unit (verifying the position of the celestial target in the VLTI field-of-view), and an optical path router (controlling the optical switchyard and the instrument feeding-optics in the VLTI laboratory). With the multiplication of VLTI instruments and their specific features (wavelength coverage, number of beams), an upgrade of ARAL (from its November 2002 version) had to be carried out: the alignment unit has been redesigned, as well as the artificial source. This source will provide a point in the visible and in J, H, K and N infrared bands, split into four beams (with a zero optical path difference at the reference position). After a description of the optomechanics and of the computer architecture of ARAL, we detail the difficulties of building an interferometric artificial source with a wide spectral range.