Near-IR Spectrograph for VSI (VLTI Spectro Imager): dispersing the light from an integrated optics beam-combiner

TL;DR

This paper details the design and implementation of a near-infrared spectrograph for the VSI interferometer, featuring an integrated optics beam-combiner capable of combining multiple telescopes with adjustable spectral resolution.

Contribution

It introduces a compact, cryo-mechanical spectrograph with an integrated optics beam-combiner for the VLTI, enabling multi-telescope interferometry with variable spectral resolution.

Findings

Successfully designed optics matching IO combiner output

Achieved spectral resolution range from R=200 to R=12000

Enabled coherent combination of 4, 6, or 8 telescopes

Abstract

We present the optical and cryo-mechanical solutions for the Spectrograph of VSI (VLTI Spectro-Imager), the second generation near-infrared (J, H and K bands) interferometric instrument for the VLTI. The peculiarity of this spectrograph is represented by the Integrated Optics (IO) beam-combiner, a small and delicate component which is located inside the cryostat and makes VSI capable to coherently combine 4, 6 or even 8 telescopes. The optics have been specifically designed to match the IO combiner output with the IR detector still preserving the needed spatial and spectral sampling, as well as the required fringe spacing. A compact device that allows us to interchange spectral resolutions (from R=200 to R=12000), is also presented.