  MISTRAL is the new Faint Object Spectroscopic Camera mounted at the folded
Cassegrain focus of the 1.93m telescope of Haute-Provence Observatory. We
describe the design and components of the instrument and give some details
about its operation. We emphasise in particular the various observing modes and
the performances of the detector. A short description is also given about the
working environment. Various types of objects, including stars, nebulae,
comets, novae, galaxies have been observed during various test phases to
evaluate the performances of the instrument. The instrument covers the range of
4000 to 8000A with the blue setting, or from 6000 to 10000A with the red
setting, at an average spectral resolution of 700. Its peak efficiency is about
22% at 6000A. In spectroscopy, a limiting magnitude of 19.5 can be achieved for
a point source in one hour with a signal to noise of 3 in the continuum (and
better if emission lines are present). In imaging mode, limiting magnitudes of
20-21 can be obtained in 10-20mn (with average seing conditions of 2.5 arcsec
at OHP). The instrument is very users-friendly and can be put into operations
in less than 15mn (rapid change-over from the other instrument in use) if
required by the science (like for Gamma-Rays Bursts). Some first scientific
results are described for various types of objects, and in particular for the
follow-up of GRBs. While some further improvements are still under way, in
particular to ease 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 e.g. the SVOM satellite and of the Rubin
telescope.
