The integral field spectrograph of SPHERE: the planet finder for VLT

SPHERE is an instrument designed and built by a consortium of French, German, Italian, Swiss and Dutch institutes in collaboration with ESO. The project is currently in its Phase B. The main goal of SPHERE is to gain at least one order of magnitude with respect to the present VLT AO facility (NACO) in the direct detection of faint objects very close to a bright star, especially giant extrasolar planets. Apart from a high Strehl ratio, the instrument will be designed to reduce the scattered light of the central bright star and subtract the residual speckle halo. Sophisticated post-AO capabilities are needed to provide maximum detectivity and possibly physical data on the putative planets. The Integral Field Spectrograph (IFS), one of the three scientific channels foreseen in the SPHERE design, is a very low resolution spectrograph (R~20) which works in the near IR (0.95-1.35 mum), an ideal wavelength range for the ground based detection of planetary features. Its goal is to suppress speckle to a contrast of 107, with a goal of 108, and at the same time provide spectral information in a field of view of about 1.5 × 1.5 arcsecs2 in proximity of the target star. In this paper we describe the overall IFS design concept