NIRSpec MOS Operations

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 and MSATA Observing ProcessJWST Multi-Object Spectroscopy 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

Most science cases that use NIRSpec MOS mode require careful alignment of science sources within the fixed grid of small 0.20" × 0.46" MSA shutters. This has a few implications:

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    A source catalog with sufficiently accurate knowledge of the astrometric positions of science sources must be available. This is required for optimal flux and wavelength calibration that depends upon accurate knowledge of a source's position within the shutter. Observed science sources cannot all be centered perfectly in their respective shutters or slitlets.
  • In order to plan optimal MOS observations and design the corresponding MSA Configurations, a specific fixed instrument Aperture Position Angle (APA) is needed.

Fixed angle observations pose significant JWST planning and scheduling constraints. For more flexible scheduling, a fixed APA will be assigned for these programs by schedulers at STScI. As a result, NIRSpec MOS observers will typically follow a two-step submission process, with a preliminary plan at initial proposal submission, and a flight-ready program update following TAC acceptance (when the plan windows and angles are assigned). The exception to this rule is for MOS programs using the MSA configured into a long slit, where the observer may wish to specify an APA using a Special Requirement in APT. For the initial proposal submission, MOS observers are encouraged to use the NIRSpec MSA Planning Tool (MPT) to create a set of placeholder MOS observations at any feasible APA of their choice. After TAC acceptance, and by the program update submission deadline, the observer will submit a program update containing executable visits using an assigned APA provided to the observer that is consistent with the plan window in the LRP.

Additionally, if a program requests pre-imaging with NIRCam in order to obtain a source catalog of sufficient astrometric accuracy, the update to flight executable visits for MOS observations will happen after the pre-images and catalogs are available. Given that the correct timing links are present in the initial proposal submission, the LRP will space out the MOS observation(s) appropriately from the pre-imaging observation to allow time for final MOS program updates to occur. 

A change to the assigned angle will be allowed if it does not result in a plan window change, as evaluated by the Program Coordinator.  If it does require a plan window change, a program change request (PCR) must be submitted to the TTRB, and will require strong science justification for approval. Such APA change requests will only be considered once for each visit, and should be made early, since they will necessitate a re-plan and technical review of the new MSA configurations.

If the MSA-based target acquisition mode MSATA is being used for the MOS observation (or for one of the other NIRSpec observing modes), target acquisition reference stars will be needed for observation planning.  However, once again, specific reference stars cannot be chosen until the observation window and APA are known. Hence, no specific stars are selected at the time of proposal submission; it is only at the program update step for accepted proposals where MSA target acquisition reference stars are selected. 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 TA modes are allowed, as is the possibility of skipping target acquisition altogether.   

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.

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