The X-ray Astronomy Satellite BeppoSAX is a joint project of the Italian Space
Agency (ASI) and the Netherlands Agency for Aerospace Programs (NIVR)
developed by a consortium of institutes in Italy and The Netherlands,
including the Space Science Department of ESA (SSD).
BeppoSAX, with an expected lifetime of 4 years, is devoted to systematic,
integrated, and comprehensive studies of galactic and extra-galactic
sources in the wide energy band 0.1-300 keV.
The spacecraft, three axis stabilised and with a total mass of 
kg,
has been launched by an Atlas G-Centaur on April 30, 1996, 04:31 GMT, into a
circular equatorial orbit of 
inclination and 600 km altitude.
The scientific payload comprises
four Narrow Field Instruments NFI (Low Energy Concentrator Spectrometer LECS,
Medium Energy Concentrator Spectrometer MECS, High Pressure Gas
Scintillation Proportional Counter HPGSPC, Phoswich Detection System PDS)
all pointing to the same direction, and two Wide Field Cameras WFC, pointing
to diametrically opposed directions perpendicular to the NFI common axis.
A detailed description of the entire BeppoSAX mission can be found in
Butler & Scarsi (1990) and in Boella et al. (1996).
The MECS, operating in the medium X-ray energy band, is one of
the NFI instruments onboard BeppoSAX.
The main scientific objectives of the MECS are:
spectroscopy from 1.3 to 10 keV (
in the range 6-16);
imaging with angular resolution at the arcmin level;
timing variability on time scales down to the millisecond.
In this paper we give a description of the MECS instrument and its
performance. Design, development, calibration
and data analysis have been carried out by the
MECS team at the IFCAI-Palermo and IFCTR-Milano Institutes in Italy,
supported by the Italian Space Agency ASI in the framework of the BeppoSAX
mission.
The MECS on-ground calibration was performed at the X-ray
PANTER facility of the Max Planck Institute, Germany; preliminary
results of on-ground calibration analysis can be found in
Boella et al. (1995) and in Molendi et al. (1995).