ISAAC: a 1- to 5-um imager/spectrometer for the VLT

ISAAC is a 1 - 5 micrometer imager/spectrometer currently under construction at ESO and scheduled for installation at one of the Nasmyth foci of the first 8 m unit telescopes of the VLT in 1998. It comprises two cameras, optimized for the 1 - 2.5 micrometer and 2 - 5 micrometer wavelength ranges, which can be used to directly view the telescope focal plane or the intermediate spectrum formed by a grating spectrometer. The complete instrument is cryogenically cooled by means of a continuous flow liquid nitrogen circuit and two closed cycle coolers and is housed in an approximately 1.5 m diameter vacuum tank permanently attached to the Nasmyth adapter/rotator. The observing modes, instrument design, construction status and performance estimated using a recently developed software simulator of the instrument are described.