The James Webb Space Telescope (JWST) is the successor mission to the Hubble Space Telescope and will
operate in the near- and mid-infrared wavelength ranges. One of the four science instruments on board the
spacecraft is the multi-object spectrograph NIRSpec, currently developed by the European Space Agency (ESA)
with EADS Astrium Germany GmbH as the prime contractor. NIRSpec will be able to measure the spectra of
more than 100 objects simultaneously and will cover the near infrared wavelength range from 0.6 to 5.0 μm at
various spectral resolutions. To verify the performance of NIRSpec and simulate future on-ground and in-orbit
observations with this instrument, the Instrument Performance Simulator (IPS) software is developed at Centre
de Recherche Astrophysique de Lyon (CRAL) as subcontractor to Astrium.
In early and mid-2009, the NIRSpec Demonstration Model (DM), fully representative up to the slit plane,
underwent cryogenic tests and calibration runs. The detector was placed at the slit plane in case of the DM to
measure specific optical performance aspects. A simplified version of the IPS was prepared, matching the DM
configuration and also serving as a testbed for the final software for the flight model. In this paper, we first
present the simulation approach used in the IPS, followed by results of the DM calibration campaign. Then, for
the first time, simulation outputs are confronted with measured data to verify their validity.