  Within the scientific goals of the THESEUS ESA/M5 candidate mission, a
critical item is a fast (within a few s) and accurate (<15 arcmin) Gamma-Ray
Burst and high-energy transient location from a few keV up to hard X-ray energy
band. For that purpose, the signal multiplexing based on coded masks is the
selected option to achieve this goal. This contribution is implemented by the
XGIS Imaging System, based on that technique. The XGIS Imaging System has the
heritage of previous payload developments: LEGRI/Minisat-01, INTEGRAL,
UFFO/Lomonosov and ASIM/ISS. In particular the XGIS Imaging System is an
upgrade of the ASIM system in operation since 2018 on the International Space
Station. The scientific goal is similar: to detect a gamma-ray transient. But
while ASIM focuses on Terrestrial Gamma-ray Flashes, THESEUS aims for the GRBs.
For each of the two XGIS Cameras, the coded mask is located at 630 mm from the
detector layer. The coding pattern is implemented in a Tungsten plate (1 mm
thickness) providing a good multiplexing capability up to 150 keV. In that way
both XGIS detector layers (based on Si and CsI detectors) have imaging
capabilities at the medium - hard X-ray domain. This is an improvement achieved
during the current THESEUS Phase-A. The mask is mounted on top of a collimator
that provides the mechanical assembly support, as well as good cosmic X-ray
background shielding. The XGIS Imaging System preliminary structural and
thermal design, and the corresponding analyses, are included in this
contribution, as it is a preliminary performance evaluation.
