  The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST),
has imaging, four coronagraphs and both low and medium resolution spectroscopic
modes . Being able to simulate MIRI observations will help commissioning of the
instrument, as well as get users familiar with representative data. We designed
the MIRI instrument simulator (MIRISim) to mimic the on-orbit performance of
the MIRI imager and spectrometers using the Calibration Data Products (CDPs)
developed by the MIRI instrument team. The software encorporates accurate
representations of the detectors, slicers, distortions, and noise sources along
the light path including the telescope's radiative background and cosmic rays.
The software also includes a module which enables users to create astronomical
scenes to simulate. MIRISim is a publicly available Python package that can be
run at the command line, or from within Python. The outputs of MIRISim are
detector images in the same uncalibrated data format that will be delivered to
MIRI users. These contain the necessary metadata for ingestion by the JWST
calibration pipeline.
