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Abstract:

<p> MOSAIC is the Multi-Object Spectrograph currently under development for the ESO Extremely Large Telescope (ELT). The instrument will provide highly multiplexed spectroscopic capabilities over a wide field of view, combining visible and near-infrared observations with both low- and high-resolution modes. The Near-Infrared (NIR) channel is one of the major subsystems of MOSAIC and is designed to support both Multi-Object Spectroscopy (MOS) and multi-Integral Field Unit (mIFU) observations over the wavelength range from 0.95 &mu;m to 1.8 &mu;m. </p> <p> The NIR channel combines complex fiber-fed architecture, cryogenic operation, high multiplexing capability and demanding spectral performance requirements. Light collected at the ELT focal plane is transported through dedicated MOS and mIFU fiber systems towards two identical cryogenic spectrographs located on the Nasmyth platform. Each spectrograph provides low-resolution, LR, (R~4,000) observations across the full NIR J and H bands wavelength range and high-resolution, HR, (R~18,000) observations in a selected H-band spectral window (1.52 – 1.62 &mu;m). </p> <p> This paper presents an overview of the NIR channel architecture, including the fiber systems, spectrographs and detectors. We summarize the main technical challenges associated with the NIR channel architecture: for the fiber links are the high multiplexing, the light injection and the routing of 2604 fibers from the ELT focal plane to the pseudo-slits at the spectrographs inside the cryostat. On the spectrograph side, the challenging elements are the optical design to accommodate both low- and high- resolution modes in a limited envelope and tight mass assignment, an extremely fast camera, the large optical components, the LR/HR mechanism, and the thermal stability of the cryostat and the cold bench. </p> <p> The current development status towards a System PDR in 2027 and an instrument-PDR with ESO in February 2028. </p> <p> The MOSAIC NIR channel is being developed by a team of seven different Research institutes and universities. This paper introduces that team and the description of the work packages developed by each of the involved MOSAIC partners. The major Management challenge is the coordination of the common design of the NIR channel where each partner is funding their own contribution (also in hardware) and the preparation of the subsystem, System and Instrument Assembly, Integration and Tests at the different sites in Europe and the verification at the ELT in Chile. </p>

Abstract:

The Extremely Large Telescope (ELT) is, to date, the most ambitious ground-based telescope under construction. MOSAIC is a multi-objects spectrograph (MOS) that aims to make full use of the largest telescope in the world. At its heart, about 250 robotic positioners will pick-off skylight from the focal surface of the ELT to feed it to its Near Infrared (NIR) and visible (VIS) spectrographs. The gigantic scale of the ELT presents three main challenges for MOSAIC positioners: (1) the light beams on the focal surface cannot be focused in a single fiber, similarly to other MOS instruments, involving a design with relay mirrors patrolling the field of view, and re-imaging the sub-field on 2 fixed fiber bundles located 600 mm behind the ELT focal plane (2) The positioner needs to adapt to the local telecentricity, which means it has to point at the ELT pupil center located 37.868 m away from the focal plane (3) The Atmospheric Dispersion Corrector (ADC) needed to cover the whole focal surface of the ELT is impossible to build to this scale; hence each positioner needs its own ADC. EPFL is responsible for designing and supervising the mass manufacturing of the positioners. This paper aims to present its initial design and prototypes.