MOS Roadmap

A general guide to preparing JWST NIRSpec multi-object spectroscopy (MOS) observations.  

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Unlike most JWST observing modes, NIRSpec MOS science observations typically follow a two-phase process. This two-phase process is due to the very tiny shutters in the MSA and the need for precise target positioning in them. After proposal submission and acceptance, an aperture position angle (APA) (Aperture PA* parameter in APT) assignment is made by STScI schedulers so that the NIRSpec MOS observations can be scheduled more efficiently and flexibly.

For most MOS science cases, observations are not specified directly in the Astronomers' Proposal Tool (APT) template in the same way as all other observing modes. The exceptions are long-slit observations or custom, manually-designed MSA configurations with the MOS and/or observations of moving targets, which will follow a modified process. Even in these exceptional cases, however, an APA assignment will often be required, demanding a two-phase approach. The MOS process is described in greater detail in the article NIRSpec MOS and MSATA Observing Process. Summarizing here:

1) In the first phase, for proposal submission, users will typically run the NIRSpec MSA Planning Tool (MPT) to create a set of planning visits with placeholder pointings and MSA shutter configurations that will be representative of the pointing and configurations after an APA has been assigned. Provided the source density is representative, MPT can predict the number of targets to expect for each pointing for a given observing strategy. For proposal submission, it is also important to include any fully specified NIRCam pre-imaging observations that will be used to locate targets for NIRSpec follow-up. Timing links should be included, to ensure that the NIRCam pre-imaging is carried out before the NIRSpec observations, by a sufficient margin

2) The second phase begins after APA assignment and after the NIRCam imaging is obtained (if any). Prior to the first MOS observing window, users will again run MPT, which will utilize the latest MSA shutter operability status, and the assigned APA, to design the final MOS observations that will specify the exact pointings and MSA configurations, and the expected targets that will be observed. This is called the program submission (a.k.a. program update). The reference stars used for MSA target acquisition must also be finalized in this submission. 

The typical MOS observer will use MPT to create optimal plans for MOS observations that attempt to maximize the number of observed sources. In order to clarify the MOS planning process, a step-by-step roadmap for proposing NIRSpec MOS science is summarized below. Additionally, some useful MOS observational strategies are discussed in the MOS Recommended Strategies article.

* 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.

Specific considerations

MPT Performance requirements

MPT has certain memory limits and computational requirements. These are given in the NIRSpec MPT - Computational Performance article.


NIRSpec observations planned with MPT require planning catalogs. Catalogs produced from HST ACS or WFC3 imaging have sufficient relative accuracy for detailed MOS observation planning and program updates. The catalog should be as complete as possible, including not only sources of scientific interest, but all other sources in the field, to be able to identify potential contaminants. The relative astrometric accuracy of the catalog is important for target acquisition to put science sources in the tiny shutters. For example, a relative astrometric accuracy of 5 mas in the catalog is necessary to deliver a NIRSpec MSA target acquisition (MSATA) accuracy of 20 mas. Catalog accuracies of 5–10 mas are required for meaningful use of the centering constraint in MPT which can help to limit slit loss and wavelength calibration uncertainties. Catalogs are not necessary for long-slit observations using the Q4 field points

NIRCam pre-imaging

NIRCam pre-imaging may be needed for MOS observations where imaging and catalogs are either non-existent or of insufficient astrometric accuracy to plan detailed MOS observations. 

NIRCam parallels

Many science cases will benefit from the ability to observe NIRSpec MOS targets while simultaneously imaging nearby fields with NIRCam for future science exploration. NIRCam parallels can be added from the NIRSpec MOS APT template. A set of joint dithers can be added to improve the NIRCam pixel sampling. The MOS Recommended Strategies article discusses this and other useful strategies.

Moving targets 

Consult the Moving Target Roadmap. MOS observations of moving targets must be specified directly in the MOS APT template at the observation level. Fill out the template directly, select a pre-designed custom config or the canned long slit, and select the correct science aperture (the Q4 field point should be used with the canned long slit). 

Canned long slit and custom MSA configurations

A pseudo long slit is offered directly in the MOS APT template in the pull-down selector for the MSA configuration. Together with the mosaic capability in the template, it is possible to step sequentially across a large extended target and obtain spectroscopy through the long slit. Similarly, users can create a custom configuration with the MSA Configuration Editor that can be used in the MOS template together with dithers specified in the MSA Configuration Editor, or mosaics specified in the MOS observation template to step across a series of smaller extended targets over the MSA field of view. 

NIRSpec MOS proposal roadmap

The following roadmap should be used by those who wish to submit a proposal for MOS observations. It follows from steps outlined in the Getting Started with JWST Proposing articleThe program update (second phase) follows a similar approach - in that case, observers will replan their MSA observations using an assigned aperture position angle (that will be populated for the user in APT) and also select reference stars for the MOS visits. Consult the NIRSpec MOS Observing Process for a details about this process.

This roadmap assumes the standard use of the MPT to define NIRSpec MOS observations using a catalog. Details specific to other more specialized cases using canned or custom MSA configurations are presented using the pull-down options shown in the roadmap at various steps.  For an example program that uses the standard roadmap, please see the NIRSpec MOS Deep Extragalactic Survey article.

Each step listed below is followed by a list of articles with additional details.

Roadmap for MOS observations (for proposal submission)

  1. Determine the range of feasible MOS observation aperture position angles (Aperture PA) for the area of interest.
    General Target Visibility Tool (GTVT)
    NIRSpec Observation Visualization Tool (NOVT)

  2. An input source catalog is required for MOS observations of multiple distinct sources with the NIRSpec MSA. Obtain or create a source Catalog of sufficient accuracy.

    1. Determine whether HST imaging and source catalogs exist for your fields. If so, obtain one for later use with the MSA Planning Tool (MPT).
      Hubble Space Telescope Finder Images and Catalogs
      Hubble Source Catalog

    2. If a catalog derived from HST imaging does not exist, then create a Catalog that spans an area of the sky in the field of interest to allow MPT to select optimal pointings. An area of 5' × 5' is suggested. For proposal submission you may use a simulated catalog or one created from observations with other telescopes.
      NIRSpec MOS Operations - Catalogs and Images
      MPT Catalogs - Examples

  3. Decide on a target acquisition strategyMOS programs prepared with MPT will use MSATA for target acquisition. However, MSATA doesn't have to be fully specified at proposal submission.
    NIRSpec Target AcquisitionNIRSpec MSA Target AcquisitionNIRSpec MSATA Reference Star Selection Recommended StrategiesNIRSpec MOS Operations - Slit Losses

    For extended sources or moving targets, it is possible to define a MOS observation in the APT template directly. For those cases, 

  4. Optional NIRCam pre-imaging may be done if existing imaging and source catalogs for your fields of interest do not provide the required positional accuracy:

     Add NIRCam Pre-Imaging to your proposal

    You should propose for NIRCam pre-imaging in the same proposal as your NIRSpec observations. NIRCam pre-imaging observations must be fully defined at the time of proposal submission.
    NIRCam Pre-Imaging

  5. Pick desired wavelength setting(s) for MOS observations. Select dispersers and filters, depending on needed wavelength coverage and resolving power.
    NIRSpec Dispersers and Filters

  6. Determine what dither strategy to use with your NIRSpec MPT-generated MOS observations. Nods and dither specifications can be added when designing an observation with the MPT Planner. (Alternatively, nods and dithers can be added using the MSA Configuration Editor for custom MOS observations described further down in the roadmap. The only options for dithering canned long slit observations is with the MSA Configuration Editor, or by using mosaicking.)
    NIRSpec Dithers and Nods
    NIRSpec Dithering Recommended Strategies

  7. Before specifying your NIRSpec MOS observation, learn about recommended practices for dithering, background subtraction, MSA leakage, and general MOS recommendations.
    NIRSpec MOS Recommended Strategies
    NIRSpec Background Recommended Strategies
    NIRSpec MSA Leakage Subtraction Recommended Strategies

  8. Determine the required exposure and detector readout parameters for your NIRSpec MOS observations including target acquisitions using the Exposure Time Calculator (ETC). Perform these calculations by bracketing the range of expected source brightnesses in the wavelength bands of interest, avoiding saturation of the brightest sources. ETC workbooks are provided for each instrument mode to illustrate parameter selection.
    NIRSpec Detector Recommended Strategies
    JWST ETC Target Acquisition

  9. Create NIRSpec MOS observation plans in APT/MPT. Run the MSA Planning Tool (MPT) in the Astronomers Proposal Tool (APT) to create automatically-designed NIRSpec MOS observations with MPT. Navigate to MPT from the Observation Folder level in APT. Follow the articles under the MOS APT Template (Catalogs, Planner, and Plans, in that order) from the links shown below to walk through the creation and evaluation of a MOS Plan. An example science case is illustrated in the article NIRSpec MOS Deep Extragalactic Survey.
    NIRSpec Multi-Object Spectroscopy APT Template
    NIRSpec MSA Planning Tool, MPT
    NIRSpec MPT - Catalogs
    NIRSpec MPT - Planner
    NIRSpec MPT - Plans
    MOS APT Template Parameters

    OR  create a MOS observation using the canned MSA Quad 4 long slit.

      (for an extended source or moving target)
    • Fill out the Astronomers Proposal Tool (APT) for your NIRSpec MOS observation directly (and completely) at the observation level in APT. (Refer to step 13). The target should be one selected from the Targets folder—your moving or extended fixed target. WATA is the default TA strategy, but you may select a different TA option. The default Science Aperture is the Q4 field point, which is used wth WATA. Specify your exposure parameters, and be sure to select the Q4 Field Point 1 Long Slit configuration to center your target in the slit at the defined reference point for that aperture. 
      MOS Custom Configuration Process

    • Optionally specify a mosaic. Canned long slit observations with the MSA can additionally make use of the mosaic option in APT to, for example, step the slit across the source in the dispersion direction. If you have a use case that requires mosaicking, you may specify the mosaic at the observations level in APT.
      APT Mosaic Planning

    • Likewise, these observations may require an Aperture PA special requirement to limit the range of position angles. Be sure to add one, if necessary (refer to step 14).

    • Continue from step 14.

    OR  create a MOS observation using custom MSA configurations.

     designed using the Manual Planner in MPT
    • Determine one or more fixed pointings and an orientation (APA). If making a custom MOS observation using a Catalog, first determine the exact pointing and orientation. It may be helpful to view an image of your target(s) in Aladin, and to overlay your catalog sources positions there. Custom MSA configurations will typically be considered placeholder observations (like those generated with MPT), unless there is a strong scientific justification for a fixed Aperture PA. After an Aperture PA is assigned, observers will be able to re-plan their custom MOS observations for the program update.
      JWST APT Aladin Viewer

    • Navigate to the MSA Configuration Editor, and create or ingest your custom MSA configurations at the fixed pointing(s) you determined. Continue to add exposure specifications using the same MSA configuration, or new MSA configurations, and include any offsets. Click Finish Plan when done.
      MOS Custom Configuration Process
      Custom MOS Observations using the MSA Configuration Editor

    • Evaluate the plan(s). These plans can be evaluated (and even merged together for making a single observation) in the Plans tab of MPT, like auto-generated MPT plans.
      NIRSpec MPT - Plans

    • Create an Observation from the plan. Select a plan, and create a MOS observation from the manual plan (refer to step 11).

    • Fill out Observation in APT. Fill out the Astronomers Proposal Tool (APT) for your NIRSpec MOS observation directly at the observation level in APT (refer to step 13).  Be sure to select the correct manually-designed MSA configuration from the pull-down menu in the exposure specification table.

    • Optionally specify a mosaic. Custom observations with the MSA may additionally make use of the mosaic option in APT to, for example, step the slit across the sources in the dispersion directionIf you have a use case that requires mosaicking, you may specify the mosaic at the observation level in APT. 
      APT Mosaic Planning

    • These observations may require an exact APA special requirement, Aperture PA.  Be sure to add one (refer to step 14).

    • Continue from step 14.
  10. The MPT should be run at several available APAs to check the variation in plan results and obtain more informed estimates of the observing time needed to execute the science. Multiplexing depends on a wide range of factors: the catalog density, extent, source distribution, the slit shape used, source centering constraints, etc. Multiplexing mostly does not depend on aperture position angles for catalogs distributed isotropically in angle. However, cases that involve a handful of highly weighted sources may be impacted by the selected angle. The user should test this in any case.
    NIRSpec MPT - Parameter Space

  11. Create an MPT Observation. Once you have an MPT plan that you are happy with, click Create Observation from the Plans pane in MPT to populate the NIRSpec MOS mode observation template with pointings, MSA configurations and exposure parameters.
    NIRSpec MPT - Plans
    NIRSpec Multi-Object Spectroscopy APT Template

  12. Note that MSA target acquisition (MSATA) reference alignment targets, called reference stars, and related TA parameters for the science observations cannot be defined at proposal submission because the execution Aperture PA isn’t yet assigned. The selection of reference stars and MSATA parameters is done at the visit level, after the MOS observations are finalized. Leave this for later, in the detailed program update submission. 

  13. Fill out Observation in APT. Navigate to the observation level of your MOS observation in APT and continue to fill out the remaining elements of the MOS APT template, including the decision to add Confirmation Images to be able to locate targets in the slitlets in post-analysis.
    NIRSpec MOS Operations - Confirmation Images

  14. Determine from viewing in Aladin, or with NOVT, whether an Aperture PA special requirement is needed. The NIRSpec MOS observation should ideally not have a specific Aperture PA special requirement added. However, some use cases may need such constraints. For scheduling flexibility, a minimum range of approximately 20°–30° is recommended. Observation planning using a catalog with MPT or a manually created plan uses an Aperture PA, however this angle is not guaranteed for the observation, nor is it used downstream to plan or schedule the observation. However, the Aperture PA assigned by STScI will be enforced for program update submission. 
    NIRSpec MOS Operations - Pre-Imaging Using NIRCam
    NIRSpec Observation Visualization Tool (NOVT)
    Aperture Position Angle Special Requirements
    APT Aladin Viewer

  15. Timing special requirement needed when NIRCam pre-imaging is requested.

    When NIRCam pre-imaging is specified in the proposal,

     Add a Timing Special Requirement to the MOS Observation

    The MOS observation should have a Timing special requirement (specifically, an AFTER <observation link>added to ensure enough time is allowed between NIRCam pre-imaging and NIRSpec follow-up observations. The recommended minimum is 60 days, but 42 days is the absolute minimum, leaving only 2 weeks for the observing team to complete program updates. The NIRSpec Observation Visualization Tool (NOVT) can be used to visualize and help plan the pre-imaging observations relative to the NIRSpec observation.

    NIRSpec MOS Operations - Pre-Imaging Using NIRCam
    Timing Special Requirements
    NIRSpec MOS and MSATA Observing Process
    NIRSpec Observation Visualization Tool (NOVT)

  16. Add optional NIRCam coordinated parallels, if desired. View the footprints of any NIRCam parallels in APT in the Aladin viewer using the FOV button and by highlighting the NIRSpec observation. Select single aperture if the view is too crowded. If the NIRCam parallels are specified with joint subpixel dither options, make sure the NIRSpec exposure time is appropriate. Some joint subpixel dither options will double or triple the number of NIRSpec dithers.
    Coordinated Parallel Observations
    Coordinated Parallels Custom Dithers
    APT Aladin Viewer

  17. The APT Visit Planner should be run to check the scheduling of the MOS visits created by the MPT.
    APT Visit Planner


Karakla, D. et al. 2014, Proc. SPIE 9149
The NIRSpec MSA Planning Tool for multi-object spectroscopy with JWST 



Latest updates

 Corrected old Manual Planner terminology, removed specifics about MOS Process timing, added a link to new article plus formatting changes 

Added link to NIRSpec MSATA Reference Star Selection Recommended Strategies