Multi-purpose InSTRument for Astronomy at Low-resolution: MISTRAL at the Observatoire de Haute-Provence.

Context. Multi-purpose InSTRument for Astronomy at Low-resolution (MISTRAL) is the new Faint Object Spectroscopic Camera mounted at the folded Cassegrain focus of the 1.93 m telescope of the Haute-Provence Observatory (OHP). Aims. We describe the design and components of the instrument and give some details about its operation. Methods. We emphasize in particular the various observing modes and the performance of the detector. A short description of the working environment is also provided. Various types of objects, including stars, nebulae, comets, novae, and galaxies, have been observed during various test phases to evaluate the performance of the instrument. Results. The instrument covers the range of 4000-8000 Å with the blue setting, or from 6000 to 10 000 Å with the red setting, at an average spectral resolution of 700. Its peak efficiency is about 22% at 6000 Å. In spectroscopy, a limiting magnitude of r ~ 19.5 can be achieved for a point source in one hour with a signal-to-noise ratio of 3 in the continuum (and better when emission lines are present). In imaging mode, limiting magnitudes of 20–21 can be obtained in 10–20 mn (with average seeing conditions of 2.5 arcsec at the OHP). The instrument is very user-friendly and can be put into operations in less than 15 mn (rapid change-over from the other instrument in use) if required by the science (e.g. for gamma-ray bursts). Some first scientific results are described for various types of objects, and in particular, for the follow-up of gamma-ray bursts. Conclusions. While some further improvements are still under way, in particular, to facilitate the switch from blue to red setting and add more grisms or filters, MISTRAL is ready for the follow-up of transients and other variable objects, in the soon-to-come era of the Space-based multi-band astronomical Variable Objects Monitor satellite and of the Rubin telescope, for instance. [ABSTRACT FROM AUTHOR]