NIRSpec MOS Operations

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 gives an overview and provides links to some important considerations for planning NIRSpec MOS observations, including target acquisition methods, dither/nods, and special considerations.

See also: NIRSpec MOS and MSATA Observing Process, JWST Multi-Object Spectroscopy Roadmap, NIRSpec MOS Recommended Strategies,  and NIRSpec Target Acquisition Recommended Strategies


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 follow a two-step submission process, with a preliminary plan at initial proposal submission, and a flight-ready program update following TAC acceptance (after the plan windows and angles are assigned). A rare 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.

In certain science cases, the PI may wish to restrict the aperture position angle up front (before angle assignment). This is generally discouraged, except in cases that scientifically warrant it.  In those cases, the preferred APA range should be added as a special requirement in APT at the time of the proposal submission.

Once assigned an angle, a program change to the assigned angle will be allowed only in limited cases without TTRB involvement, as evaluated by the Program Coordinator. Outside of those limited cases, a program change request (PCR) must be submitted to the TTRB with 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 a program requests pre-imaging with NIRCam in order to obtain a source catalog of sufficient astrometric accuracy for the MOS observation, the update to flight executable visits for MOS observations must wait until the pre-images and catalogs are available. The MOS and pre-imaging observations require the correct timing links in the initial proposal submission, to separate the observations appropriately to allow time for final MOS program updates to be made.

If the MSA-based target acquisition mode MSATA is being used for the MOS observation, target acquisition reference sources will be needed for observation planning.  Specific reference sources cannot be chosen until the observation window and APA are known. Hence, no specific sources need be selected at the time of proposal submission; it is only at the program update step for accepted proposals where MSA target acquisition reference sources are selected. This TA method is designed for use with catalogs having coordinate accuracy between 5 and 50 mas. For science cases that do not have strict astrometric requirements, other TA options (with lower overhead) are available. As described in the NIRSpec Target Acquisition article, WATA and VERIFY_ONLY TA modes are allowed, and the possibility of skipping target acquisition altogether.   

Most of the above considerations are described in more detail in the following article:  

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.



Notable updates
  •  
    Wording changes for clarity and to correct information on angle changes.

  •  
    Clarified recommendations about adding an APA special requirement, but only if necessary.

  •  
    Added information about assigned angle changes


  • Added link to NIRSpec MSATA Reference Star Selection Recommended Strategies article
Originally published