The design of the MOONS-VLT spectrometer

MOONS is a new conceptual design for a multi-object spectrograph for the ESO Very Large Telescope (VLT) which will provide the ESO astronomical community with a powerful, unique instrument able to serve a wide range of Galactic, Extragalactic and Cosmological studies. The instrument foresees 1000 fibers which can be positioned on a field of view of 500 square-arcmin. The sky-projected diameter of each fiber is at least 1 arcsec and the wavelengths coverage extends from 0.8 to 1.8 μm. This paper presents and discusses the design of the spectrometer, a task which is allocated to the Italian National Institute of Astrophysics (INAF). The baseline design consists of two identical cryogenic spectrographs. Each instrument collects the light from over 500 fibers and feeds, through dichroics, 3 spectrometers covering the "I" (0.79-0.94 μm), "YJ" (0.94-1.35 μm) and "H" (1.45-1.81 μm) bands. The low resolution mode provides a complete spectrum with a resolving power ranging from R'4,000 in the YJ-band, to R'6,000 in the H-band and R'8,000 in the I-band. A higher resolution mode with R'20,000 is also included. It simultaneously covers two selected spectral regions within the J and H bands.