The Centre de Recherche Astrophysique de Lyon (CRAL) has recently developed two instrument simulators
for spectrographic instruments. They are based on Fourier optics, and model the whole chain of acquisition,
taking into account both optical aberrations and diffraction effects, by propagating a wavefront through the
instrument, according to the Fourier optics concept. One simulates the NIRSpec instrument, a near-infrared
multi-object spectrograph for the future James Webb Space Telescope (JWST). The other one models the
Multi Unit Spectroscopic Explorer (MUSE) instrument, a second-generation integral-field spectrograph for the
Very Large Telescope (VLT). The two simulators have been developed in different contexts (subcontracted
versus developed internally), and for very different instruments (space-based versus ground-based), which
strengthen the CRAL experience. This paper describes the lessons learned while developing these simulators:
development methods, phasing with the project, points to focus on, getting data, interacting with scientists
and users, etc.
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.
NIRSpec is the near-infrared multi-object spectrograph for the future James Webb Space Telescope (JWST). It is
developed by EADS Astrium for the European Space Agency. The Centre de Recherche Astrophysique de Lyon (CRAL)
has developed the Instrument Performance Simulator (IPS) software that is being used for the modeling of NIRSpec's
performances and to simulate raw NIRSpec exposures. In this paper, we present the IPS software itself (main simulation
modules and user's interface) and discuss its intrinsic accuracy. We also show the results of simulations of calibration
exposures as they will be obtained during the NIRSpec on-ground calibration campaign.
The future James Webb space telescope (JWST), developed jointly by the American, European and Canadian space
agencies (NASA, ESA and CSA), is scheduled for launch in 2013. Among its instrument suite, the spectrograph
NIRSpec will provide astronomers with multi-object, integral-field and classical slit spectrographic capabilities in the
near-infrared (0.6-5.0 μm). NIRSpec is being built by EADS Astrium for ESA and it was quickly realized that given the
complexity of the instrument, it was necessary to develop dedicated software for the modeling of its performances. In
this context, the Centre de Recherche Astrophysique de Lyon (CRAL) is responsible of the development of the so-called
NIRSpec instrument performance simulator (IPS) that will serve as a basis for early performance verification purposes;
provide inputs and support for the verification and calibration campaigns, as well as for the development of the
instrument calibration, target acquisition and data reduction procedures.
In this paper, we present the IPS software itself, emphasizing its capability to generate simulated NIRSpec detector
exposures for the various modes of the instrument (multi-object, integral field unit, fixed slits) and for a large variety of
situations (test, calibration, scientific observations...). We will also show simulations results.