Instrument physical model for the SOXS (Son Of X-Shooter) spectrograph

We present the proposal for the physical instrument model of the SOXS (Son OF X-Shooter) spectroscopic facility mainly devoted to the follow-up observation of transient sources. A dedicated suitable instrument to exploit the science of these transients is lacking, resulting in severe science "dissipation". SOXS will cover the optical/NIR band (0.35-1.75 μm) with a medium resolution (R 4500), down to the limiting magnitude of R 20-20.5 (1 hr at S/N 10) that is perfectly suited to study transients from on-going imaging surveys. Imaging capabilities in the optical are also foreseen to allow for multi-band photometry of the faintest transients with a field of view of at least 2arcmin. We propose to implement a physical modelling approach in order to link the instrument parameters and behaviour to physical quantities, thus providing a description of the instrument that can be connected with measurements. The method has been already successfully applied to the X-shooter instrument. The X-shooter physical model is based on a kernel optical ray-tracing realised by means of matrix optics representation, which can handle a large number of wavelengths. This can be extended to the SOXS design. The foreseen applications of the SOXS physical model are broad, ranging from support to detailed instrument design and development of the data reduction software, wavelength calibration, evaluation of instrument performance as a function of the model parameters, instrument alignment, and support during the commissioning phase and as a tool for quality check during operations.