The Near Infrared Spectrograph (NIRSpec) instrument is one of the four scientific instruments aboard the James Webb Space Telescope (JWST). NIRSpec can be operated in Multi-Object Spectroscopy (MOS), Fixed- slit Spectroscopy (FS), and Integral Field Spectroscopy (IFS) modes; with spectral resolutions from 100 to 2700. Two of these modes, MOS and IFS, share the same detector real estate and are mutually exclusive. Consequently, the micro-shutters used to select targets in MOS mode must all be closed when observing in IFS mode. However, due to the finite contrast of the micro-shutter array (MSA), some amount of light passes through them even when they are commanded closed. This light creates a low, but potentially significant, parasitic signal, which can affect IFS observations. Here, we present the work carried out to study and model this signal. Firstly, we show the results of an analysis to quantify its levels for all NIRSpec spectral bands and resolution powers. We find a level of parasitic signal that is, in general, lower than 10% of the incident, extended IFS signal. We also show how these results were combined with signal-to-noise considerations to help consolidate the observation strategy for the IFS mode and to prepare guidelines for designing observations. In general, we find that this parasitic signal will be less than the statistical noise of a Zodiacal light exposure up to ∼40 groups for the NIRSpec grating configurations, and ∼10 groups for the prism configuration. In a second part, we report on the results of our work to model and subtract this signal. We describe the model itself, its derivation, and its accuracy as determined by applying it to ground test data. 

The Near-Infrared Spectrograph (NIRSpec) is one of four instruments aboard the James Webb Space Telescope (JWST). NIRSpec is developed by ESA with AIRBUS Defence & Space as prime contractor. The calibration of its various observing modes is a fundamental step to achieve the mission science goals and provide users with the best quality data from early on in the mission. Extensive testing of NIRSpec on the ground, aided by a detailed model of the instrument, allow us to derive initial corrections for the foreseeable calibrations. We present a snapshot of the current calibration scheme that will be revisited once JWST is in orbit.

The Near-Infrared Spectrograph (NIRSpec) is one of four instruments aboard the James Webb Space Telescope (JWST). NIRSpec is developed by ESA with AIRBUS Defence & Space as prime contractor. The calibration of its various observing modes is a fundamental step to achieve the mission science goals and provide users with the best quality data from early on in the mission. Extensive testing of NIRSpec on the ground, aided by a detailed model of the instrument, allow us to derive initial corrections for the foreseeable calibrations. We present a snapshot of the current calibration scheme that will be revisited once JWST is in orbit.

The Near-Infrared Spectrograph (NIRSpec) is one of the four science instruments onboard the James Webb Space Telescope (JWST). The instrument features a focal plane array (FPA) consisting of two 2048 x 2048 HAWAII-2RG sensor chip assemblies (SCAs) with a cutoff wavelength of approximately 5.3 μm. The detectors are read out via a pair of SIDECAR ASICs. To ensure a stable operating environment and best performance, the FPA is temperature controlled via a dedicated control loop by the NIRSpec focal plane electronics. The targeted in-orbit operating temperature of the NIRSpec FPA is close to 42.8 K.

Due to the low background levels that the JWST will provide, most NIRSpec observations of very faint targets will be detector noise limited. Therefore, stringent noise requirements on the detector system were put in place. In order to meet these requirements, NIRSpec offers a dedicated readout mode for its detectors that is called improved reference sampling and subtraction (IRS2).

In this paper we present the noise performance of the NIRSpec detectors as a function of readout mode and exposure parameters. We find that the NIRSpec detector system meets its stringent noise requirement of 6 electrons total noise in a ~1000 second exposure. We also highlight the types and effects of different kinds of bad pixels that are present in the detectors in small numbers. 

JWST/NIRSpec will include the first space-borne multi-object spectrograph, comprising a micro-shutter array (MSA) of a quarter of a million closable apertures that can be individually addressed to select up to a couple of hundred objects within a ~3.2x3.4 arcmin field of view. Although more than ~85% of the unvignetted shutters are fully operational, the high degree of mechanical movement combined with complex circuitry on a small scale, inevitably leads to some non-operable shutters.

In this paper we present an overview of the operability assessment concept for the MSA, employed during both ground tests and in flight. We describe the procedures used to detect, mitigate against, and even repair the non-operable shutters, and show the effect upon the multiplexing capability and output data from NIRSpec. We also present the operability trending results from ground tests, and discuss the probable impact on nominal operations after launch.