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The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-raycoded-aperture mask instrument with a primary field of view of 4.6 x 4.6degrees (FWHM). The instrument collimators become increasingly transparent atenergies above $\sim$100 keV, making CZTI sensitive to radiation from theentire sky. While this has enabled CZTI to detect a large number of off-axistransient sources, calculating the source flux or spectrum requires knowledgeof the direction and energy dependent attenuation of the radiation incidentupon the detector. Here, we present a GEANT4-based mass model of CZTI andAstroSat that can be used to simulate the satellite response to the incidentradiation, and to calculate an effective "response file" for converting thesource counts into fluxes and spectra. We provide details of the geometry andinteraction physics, and validate the model by comparing the simulations ofimaging and flux studies with observations. Spectroscopic validation of themass model is discussed in a companion paper, Chattopadhyay 2021.

The AstroSat Mass Model: Imaging and Flux studies of off-axis sources with CZTI