VLTI Spectro Imager
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4. Instrument concept

Friday 18 January 2008

The phase A study has led to an instrument concept consisting of:
- Integrated optics multi-way beam combiners providing high-stability visibility and closure-phase measurements on multiple baselines;
- A cooled spectrograph providing resolutions between R=100 and R=12000 over the J, H, or K bands;
- An integrated high-sensitivity switchable H/K fringe tracker capable of real-time cophasing or coherencing of the beams from faint or resolved sources; \item Hardware and software to enable the instrument to be aligned, calibrated and operated with minimum staff overhead.

These features act in synergy to provide a scientific capability which is a step beyond existing instruments. Compared to the single closure phase measured by AMBER, the 3 independent closure phases available by VSI4, the 10 independent closure phases measured by VSI6 and the 21 independent closure phases measured by VSI8 will make true interferometric imaging, as opposed to simply measuring visibilities, a routine process at the VLTI. The capability to cophase on targets up to K=10 will allow long integrations at high spectral resolutions for large classes of previously inaccessible targets, and the capability to do self-referenced coherencing on objects as faint as K=13. It will allow imaging of >99% of targets for which no bright reference is sufficiently close by. VSI will be able to provide spectrally and spatially-resolved ``image cubes’’ for an unprecedented number of targets at unprecedented resolutions.

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General implementation of the VSI instrument

A system analysis of VSI has allowed the high level specifications of the system to be defined, the external constraints to be clarified and the functional analysis to be performed. The system design features 4 main assemblies: the science instrument (SI), the fringe tracker (FT), the common path (CP) and the calibration and alignment tools (CAT). The global implementation is presented in the Figure above.

The optics design of the science instrument features beam combination using single mode fibers, an integrated optics chip and 4 spectral resolutions through a cooled spectrograph. The common path includes low-order adaptive optics (with the current knowledge reduced to only tip-tilt corrections). VSI also features an internal fringe tracker. These servo-loop systems relax the constraints on the VLTI interfaces by allowing for servo optical path length differences and optimize the fiber injection of the input beams to the required level. An internal optical switchyard allows the operator to choose the best configuration of the VLTI co-phasing scheme in order to perform phase bootstrapping for the longest baseline on over-resolved objects. Three infrared science detectors are implemented in the instrument, one for the Science Instrument, one for the fringe tracker, and one for the tip-tilt sensor. The instrument features 3 cryogenic vessels.

An important part of the instrument is the control system which includes several servo-loop controls and management of the observing software. The science software manages both data processing and image reconstruction since one of the products of VSI will be a reconstructed image like for the millimeter-wave interferometers. The instrument development includes a plan for assembly, integration and tests in Europe and in Paranal.

The instrument preliminary analysis report discusses several important issues such as the comparison between the integrated optics and bulk optics solutions, the standard 4- and 6-telescope VLTI array for imaging, the proposed implementation of M12 mirrors to achieve these configurations with VSI4 and VSI6, implication of using an heterogeneous array and analysis of the thermal background.

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