The Joint European X-ray Telescope (JET-X) is one of the core scientific instruments of the SPECTRUM RONTGEN-γ astrophysics mission. The project is a collaboration of British, Italian and Russian consortia, with the participation of the Max Planck Institut (Germany). JET-X was designed to study the emission from X-ray sources in the band of 0.3-10 keV. Citterio et al. (1996 and references therein) describe its structure, composed by two identical and coaligned Wolter I telescopes. Focal plane imaging is provided by cooled X-ray sensitive CCD detectors which combine high spatial resolution with good spectral resolution, including coverage of the iron line complex around 7 keV at a resolution of ΔE/E ~ 2%.