The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope V. Optimal algorithms for planning multi-object spectroscopic observations

TL;DR

The paper presents eMPT, a modular software suite for planning JWST/NIRSpec MOS observations using the MSA, addressing the complexity of matching ~249,660 shutters to a prioritized target catalog. It introduces key algorithms, notably the matrix (Arribas) algorithm for non-overlapping spectral packing and the initial pointing algorithm (IPA) for maximizing PC1 target coverage, while accounting for instrument distortion, shutter operability, and spectral overlaps. The toolkit offers extensive capabilities beyond the standard MPT, including dynamic PRISM spectrum handling, contamination screening, sky-background optimization, and batch scripting, all designed to produce APT-compatible outputs. This work enhances observational efficiency and scientific return by enabling optimized, customizable, multi-exposure MOS configurations for NIRSpec, with practical impact on planning and proposal success for JWST programs.

Abstract

We present an overview of the capabilities and key algorithms employed in the so-called eMPT software suite developed for planning scientifically optimized, multi-object spectroscopic (MOS) observations with the Micro-Shutter Array (MSA) of the Near-Infrared Spectrograph (NIRSpec) instrument on board the James Webb Space Telescope (JWST), the first multi-object spectrograph to operate in space. NIRSpec MOS mode is enabled by a programmable MSA, a regular grid of ~250,000 individual apertures that projects to a static, semi-regular pattern of available slits on the sky and makes the planning and optimization of an MSA observation a rather complex task. As such, the eMPT package is offered to the NIRSpec user community as a supplement to the MSA Planning Tool (MPT) included in the STScI Astronomer's Proposal Tool (APT) to assist in the planning of NIRSpec MOS proposals requiring advanced functionality to meet ambitious science goals. The eMPT produces output that can readily be imported and incorporated into the user's observing program within the APT to generate a customized MPT MOS observation. Furthermore, its novel algorithms and modular approach make it highly flexible and customizable, providing users the option to finely control the workflow and even insert their own software modules to tune their MSA slit masks to the particular scientific objectives at hand.

Figures (6)

• Figure 1: Map of the angular field distortion displayed by the NIRSpec collimator optics. The best-fit linear model is shown in red. The actual distortion, amplified by a factor of 25 for clarity, is plotted in black.
• Figure 2: Schematic illustration of the vertical separation, $\Delta S_{\!D}$, between two spectra from two shutters separated horizontally by $\Delta i$ when the disperser is mis-clocked by the angle $\Delta\phi_z$
• Figure 3: Maps of how many shutters to the left and right along an MSA row are required for PRISM spectra to avoid overlapping horizontally.
• Figure 4: Schematic illustrating the logic behind the digital shift-vector map employed in the IPA. Each rectangle shown represents the acceptance zone of a viable shutter (identified by its MSA coordinates) in shift-vector space, accessible to the associated target (red for target A and blue for target B) within a given search box. The (green) area of overlap of these acceptance zone areas in shift-vector space contains the shift vector value required from the nominal pointing position to simultaneously observe these two targets in a single MOS observation.
• Figure 5: Example of an MSA PRISM exposure obtained during NIRSpec commissioning in which the matrix algorithm, provided with the bare viable slitlet map as input, was used to produce an optimized MSA mask that yields a total of 235 non-overlapping and non-truncated MSA and 5 fixed-slit PRISM spectra of the diffuse interstellar medium emission during a blind pointing toward the field near the Galactic center.