VLTI Spectro Imager
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1. General context

Friday 18 January 2008

At the beginning of the 21st century, infrared observations performed at the milli-arcsecond scale are essential for many astrophysical investigations either to compare the same physical phenomena at different wavelengths (like sources already observed with the VLBI or soon to be observed by ALMA) or to get finer details on observations carried out with the Hubble Space Telescope (HST) or 10-m class telescopes equipped with adaptive optics. The astrophysical science cases at milli-arcsecond scales which cover from planetary physics to extragalactic studies can only be studied using interferometric aperture synthesis imaging with several optical telescopes. In this respect, the Very Large Telescope (VLT) observatory of the European Southern Observatory (ESO) is a unique site world-wide with 4x8-m unit telescopes (UTs), 4x1.8-m auxiliary telescopes (ATs) and all the required infrastructure, in particular delay lines (DLs), to combine up to 6 telescopes. The VLT Interferometer (VLTI) infrastructure can be directly compared to the Plateau de Bure Interferometer (PdBI) which combines 6x15-m antenna over 500-m in the millimeter-wave domain. The quality of the foreseen images can be directly compared to the images provided by the PdBI. However, the angular resolution of the VLTI is a few hundred times higher due to the observation at shorter wavelengths. The large apertures of the VLTI telescopes and the availability of fringe tracking allow sensitivity and spectral resolution to be added to the imaging capability of the VLTI.

In April 2005, at the ESO workshop on ``The power of optical/infrared interferometry: recent scientific results and second generation VLTI instrumentation’’, two independent teams have proposed two different concepts for an imaging near-infrared instrument for the VLTI: BOBCAT and VITRUV. In October 2005, the science cases of these instruments were approved by the ESO Science and Technical Committee. In January 2006, the two projects merged in order to propose the VLTI spectro-imager (VSI) as a [response to the ESO call for phase A proposals for second generation VLTI instruments->doc35]. The phase A study ended in September 2007 after an ESO board review.

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