  MIRI, the Mid-InfraRed Instrument, is one of four instruments being built for
the James Webb Space Telescope. It is being developed jointly between an
European Consortium (21 institutes from 10 countries, under the auspices of
ESA), and the US. MIRI consists of an imager, a coronograph, a low-resolution
spectrograph, and an Integral Field Unit (IFU) Medium Resolution Spectrometer
(MIRI-MRS). The latter will be the first mid-infrared IFU spectrograph, and one
of the first IFUs to be used in a space mission. To give the MIRI community a
preview of the properties of the MIRI-MRS data products before the telescope is
operational, the Specsim tool has been developed to model, in software, the
operation of the spectrometer. Specsim generates synthetic data frames
approximating those which will be taken by the instrument in orbit. The program
models astronomical sources and transforms them into detector frames using the
predicted optical properties of the telescope and MIRI. These frames can then
be used to illustrate and inform a range of operational activities, including
data calibration strategies and the development and testing of the data
reduction software for the MIRI-MRS. Specsim will serve as a means of
communication between the many consortium members by providing a way to easily
illustrate the performance of the spectrometer under different circumstances,
tolerances of components and design scenarios.
