Curved detector-based optical design for the VLT/BlueMUSE instrument

BlueMUSE (Blue Multi Unit Spectroscopic Explorer) is a blue-optimised, medium spectral resolution, panoramic integral field spectrograph proposed for the Very Large Telescope (VLT) and based on the MUSE concept. BlueMUSE will open up a new range of galactic and extragalactic science cases allowed by its specific capabilities in the 350 - 580 nm range: an optimised end-to-end transmission down to 350 nm, a larger FoV (up to $1.4 \times 1.4$ arcmin$^2$) sampled at 0.3 arcsec, and a higher spectral resolution ($\lambda/\Delta \lambda \sim 3500$) compared to MUSE. To our knowledge, achieving such capabilities with a comparable mechanical footprint and an identical detector format ($4\text{k} \times 4\text{k}$, 15 $\mathrm{\mu m}$ CCD) would not be possible with a conventional spectrograph design. In this paper, we present the optomechanical architecture and design of BlueMUSE at pre-phase A level, with a particular attention to some original aspects such as the use of curved detectors.