NIRSpec MOS Operations

The operational features of the JWST NIRSpec multi-object spectroscopy (MOS) observing mode define how instrument-level capabilities can be applied for science. These features include target acquisition methods, dither/nods for each observing mode, and special considerations.

See also: NIRSpec MOS Observing ProcessMOS RoadmapNIRSpec MOS Recommended Strategies,  and NIRSpec Target Acquisition Recommended Strategies

The NIRSpec MOS observing mode provides the means to simultaneously obtain spectra of multiple objects within the 3.6' × 3.4' field of view of the micro-shutter assembly (MSA). This observing mode has some unique operational aspects that differ from other JWST observing modes. This article discusses and links to some important considerations for planning NIRSpec MOS observations.

MOS operations highlights

In most science cases using NIRSpec MOS mode, accurate alignment of science sources within the small 0.2" × 0.46" MSA shutters requires that observations must be planned and executed at a fixed instrument Aperture Position Angle (APA)*. These fixed observations translate to significant JWST planning and scheduling constraints. As a result, users seeking to acquire NIRSpec MOS observations will typically follow a multi-step observing process with proposal and flight-ready update stages, instead of the single stream process available to most science modes. The exception to this rule is for MOS programs using the MSA configured to a long slit, where the APA is not fixed.

For proposals that require accurate placement of sources within MSA shutters, MOS observers are encouraged to use the NIRSpec MSA Planning Tool (MPT) to create the required placeholder observations. After a MOS proposal is accepted by the JWST time allocation committee (TAC), the flight update to executable visits will be carried out using an orientation assigned by the JWST observing cycle long range plan. Additionally, if a program requests the pre-imaging option with NIRCam, the update to flight executable visits for MOS observations will happen after the pre-images and catalogs are acquired and delivered. 

The NIRSpec MSA is different from many existing ground-based MOS instruments because of the fixed grid of small-sized shutters (0.2" × 0.46"). This means that observed science sources cannot all be centered in their respective slitlet apertures. Hence, for optimal flux and wavelength calibration, the NIRSpec MOS observations demand the accurate relative knowledge of astrometric positions of science sources and target acquisition reference objects for observation planning. Therefore, the MSA-based NIRSpec target acquisition (MSATA) uses reference stars to align science sources in their respective slitlets (or columns of open shutters that form spectral slits). This TA method is designed for use with catalogs having coordinate accuracies between 5 and 50 mas. For science cases that do not have strict astrometric requirements, such as a long slit of MSA shutters, other TA options (with lower overhead) are available. As described in the NIRSpec Target Acquisition article, WATA and VERIFY_ONLY are allowed, as is the possibility of skipping TA. 

Not all science fields have excellent astrometry (e.g. from HST) for MSATA, so the concept of using NIRCam to acquire pre-images was developed for follow-up NIRSpec MOS observation planning.   

Additional information on operational aspects of the NIRSpec MOS observing mode are included in the following articles:

To get the most out of NIRSpec multiplexing, the JWST Astronomers Proposal Tool  (APT) includes a specialized MSA Planning Tool (MPT). The MPT has algorithms to search through multiple field pointings to derive optimized MOS observing plans over multiple different MSA shutter configurations, including dithers and weight priorities on prime science sources of interest. The NIRSpec MPT suite of observation planning articles were created to assist users in making successful NIRSpec MOS observations.

Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.



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