A general guide to preparing JWST NIRSpec multi-object spectroscopy (MOS) observations is provided in this article.

For most MOS science cases, observations are not specified directly in the Astronomers' Proposal Tool (APT) template in the same way as other observing modes. The typical MOS observer will use the NIRSpec MSA Planning Tool (MPT) to create optimal plans for MOS observations that attempt to maximize the number of observed sources. 

Unlike most JWST observing modes, NIRSpec MOS science observations typically follow a two-phase process. This two-phase process is a result of the need for precise target positioning in very tiny shutters in the MSA. After proposal submission and acceptance, an aperture position angle assignment (called Assigned Aperture PA in APT)  is made by STScI schedulers so that the NIRSpec MOS observations can be scheduled more efficiently and flexibly. All programs using MSATA, or which have MOS observations that use an MSA Catalog Target, will follow the two-phase submission process. The nominal MOS process is described in greater detail in the NIRSpec MOS and MSATA Observing Process article. Summarizing here:

(1) In the first phase, "proposal submission", observers 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 of the catalog 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. 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.

(2) The second phase begins after an Aperture PA assignment by STScI and after any NIRCam pre-imaging is obtained. Prior to the first MOS observing window, observers will first update the catalog from any pre-imaging. The placeholder observations/visits will then need to be updated by running MPT once again, utilizing the latest MSA shutter operability information and the Assigned Aperture PA to design the final MOS program. These observations will specify the exact pointings and MSA configurations, and the expected targets that will be observed. This is called the "MOS program update submission". The reference stars used for MSA target acquisition must also be finalized in this submission. The roadmap for planning MOS program updates is described below, following the section on MOS proposals.

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Some preliminary notes

Catalogs

NIRSpec observations planned with MPT typically 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 within the slits of science sources. The relative astrometric accuracy of the catalog (including both science sources and reference stars) is important for executing MSATA target acquisition, and to ultimately put science sources in the tiny shutters.  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. If this is the case, observers will use the updated MSA catalog derived from the pre-imaging to refine their MOS observations for the MOS program update submission.

NIRCam or MIRI 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. Likewise, parallel observations with MIRI could enable expanded science goals. Coordinated parallels with MIRI or NIRCam 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 

Observers with moving targets should consult the generic JWST 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 pre-defined long slit, and select the correct science aperture (the Q4 field point should be used with the matching long slit MSA configuration). 

Pre-defined 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. Using the dispersion offset values in the configurations/pointings area of the observation template, it is possible to step the slit 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 to step across a series of smaller extended targets over the MSA field of view. 

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NIRSpec MOS proposal roadmap

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

Roadmap for MOS observations (for proposal submission)

1. Determine the range of feasible MOS observation aperture position angles (Aperture PA) for the field 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
      MAST
      Vizier
      
      

   2. If a catalog derived from HST imaging does not exist, then create a fake 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. The fake catalog should have a similar density and extent to the one that is expected from pre-imaging, in order to get realistic estimates of observing efficiency with different planning strategies.
      NIRSpec MOS Operations - Catalogs and Images
      NIRSpec MPT - Catalogs
      MPT Catalogs - Examples
      
      

3. Decide on a target acquisition strategy based on the positional accuracy needed for your MOS science goals. MOS programs prepared with MPT that use a catalog should use MSATA for target acquisition. However, MSATA doesn't have to be fully specified at proposal submission.
   NIRSpec Target Acquisition
   NIRSpec MSA Target Acquisition
   NIRSpec MOS Operations - Slit Losses

   For extended sources or moving targets, a relaxed accuracy for target acquisition may be possible. For those cases, 

   WATA or VERIFY_ONLY may be used

   NIRSpec Wide Aperture Target Acquisition
   NIRSpec Verify Only Target Acquisition
   
   The MOS observations in these programs can be specified in the APT template directly (without the use of MPT). The MOS observation must be finalized at proposal submission. If a particular orientation is needed, a special requirement restricting the aperture position angle is warranted and should be added to the observation in the proposal.

4. Determine whether NIRCam pre-imaging is required if existing imaging and source catalogs for your fields of interest do not provide the required positional accuracy for the TA or the science.

   First, check the absolute and relative astrometric accuracy of your catalog coordinates, and determine if the accuracy delivered by MSATA is sufficient for your science goals. Imaging data used to derive the astrometry for your catalog of reference stars and targets needs to be carefully vetted against Gaia stars. In particular, the MSA Target Acquisition procedure has a very limited ability to correct for errors in the initial roll, and so it is strongly recommended that the rotation of the reference stars on the sky be aligned with that of the Gaia frame to better than a half arc-minute in rotation.
   NIRSpec MOS Operations - Slit Losses

   If needed, 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

   To add NIRCam Pre-Imaging to your proposal

   • Choose wavelength setting(s) for the NIRCam pre-imaging observation. It is sometimes desirable to use the F115W filter which is closely matched to the NIRSpec F110W TA filter, for example.
     NIRCam Filters
     
     
   • Determine which dithers and mosaic patterns to use with your NIRCam imaging. NIRCam pre-imaging observations should be fully defined at the time of proposal submission, and should cover a sufficiently large field of view to allow for spectroscopic follow-up at any potential assigned Aperture PA. An area of 5' × 5' is suggested. The NOVT tool is recommended for visualizing the NIRCam and NIRSpec observations together.
     NIRCam Dithers and Mosaics
     NIRSpec Observation Visualization Tool (NOVT)
     NIRCam Primary Dithers
     NIRCam Mosaics
     NIRCam Dithers and Mosaics
     NIRCam Imaging Recommended Strategies
     
     
   • Determine the required exposure time and detector readout parameters for your NIRCam observation including target acquisition using the Exposure Time Calculator (ETC).
     JWST ETC New User Guide
     JWST ETC Workbooks Overview
     JWST ETC Target Acquisition
     NIRCam Imaging Recommended Strategies
     
     
   • Complete the Astronomers Proposal Tool (APT) template for your NIRCam pre-imaging observation. 
     NIRCam imaging template in APT
5. Pick desired wavelength setting(s) for your MOS observations. Select dispersers and filters, depending on the wavelength coverage and the resolving power required.
   NIRSpec Dispersers and Filters
   
   
6. Determine what dither strategy to use with your NIRSpec MPT-generated MOS observations. Nods and dithers are specified when designing an observation with the MPT Planner, and will appear in the observation template. (Alternatively, nods and dithers can be added directly by specifying them in the Pointings/Configurations table in the observation template, or in the MSA Configuration Editor as described in step 8 for custom MOS observations. Dithers for pre-defined long slit observations can be specified in any of these ways.)
   NIRSpec Dithers and Nods
   
   

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 Dithering Recommended Strategies
   NIRSpec Background Recommended Strategies
   NIRSpec MSA Leakage Subtraction Recommended Strategies

   For moving targets

   JWST Moving Targets in APT
   Moving Target Observing Strategies

8. Create NIRSpec MOS observation plans in APT/MPT. Run the MSA Planning Tool (MPT) in the Astronomers Proposal Tool (APT) to create an automatically-designed NIRSpec MOS observation with MPT. Start by navigating to MPT by clicking the MPT icon in the toolbar of 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 MSA Planning Tool, MPT
   NIRSpec Multi-Object Spectroscopy APT Template
   NIRSpec MPT - Catalogs
   NIRSpec MPT - Planner
   NIRSpec MPT - Plans

   OR  create a MOS observation using the pre-defined MSA Quad 4 long slit

    for an extended source or moving target

   • Your NIRSpec MOS long slit observation can be specified directly In the Astronomers Proposal Tool (APT) template. The target of the observation should be a moving target or extended fixed target selected from the Targets folder in APT.
     
     
   • WATA is the default TA strategy, but you may select a different TA option (VERIFY_ONLY or NONE). None of these options use reference stars.
     
     
   • The default Science Aperture for the long slit is the Q4 field point 1. Specify your exposure parameters, and be sure to select the Q4 Field Point 1 Long Slit configuration together with the corresponding Q4 Field Point science aperture to center your target in the slit at the defined reference point for that aperture. 
     Long Slit MOS Observations
     
     
   • If desired, specify additional exposures with different dispersion offsets in the Pointings/Configurations table of the Science Parameters section of the MOS template. Using the same long slit configuration, these offsets can be used to step the slit across an extended source in the dispersion direction, for example. The offsets are in units of shutters, and the MSA shutter pitch is ~0.27 arcsec.
     MOS Recommended Strategies
     
     
   • For the initial exposure, check that the target is centered in the long slit at the defined reference point and Assigned Aperture PA. The long slit associated with Q4 Field Point 1 is located at column 250 in Q4 (quadrant 4). For a fixed source, display an image of the source in Aladin, and highlight the observation to see the footprint of the MSA. Select the FoV icon to see the 4 quadrants of the MSA. The target should be centered near the top left of Q4.
     APT Aladin Viewer
     
     
   • Fill out the remainder of the MOS Observation template in APT (refer to step 13).
     
     
   • These observations will likely require an Aperture PA special requirement to limit the range of position angles. Be sure to add one, if needed (refer to step 15). MOS observations of moving targets may need added special requirements to limit the timing in order to place aperture position angle constraints based on the orientation of the target. These observations will follow the single-stream process, so observers will not have a chance to replan at a later time.
     
     
   • Continue from step 12 onward.

   OR  create a MOS observation using custom MSA configurations

   designed with a Catalog using the MSA Configuration Editor

   • If making a custom MOS observation using a catalog, first select an exact pointing and orientation. It may be helpful to view an image of your target(s) in Aladin, and to overlay your MSA Catalog Target source positions there.
     JWST APT Aladin Viewer
     
     
   • When returning from Aladin, click the Form Editor icon to be able to see the MOS observation template.
     
     
   • MOS observations that use a catalog will need to use the MSATA target acquisition method. These observations will later get an assigned APA after program acceptance and will follow the two-phase process that will provide an opportunity to re-plan the observation in a program update after an APA assignment.
     NIRSpec MSA Target Acquisition
     
     
   • Navigate to the MSA Configuration Editor, and create or import your custom MSA configuration planned at a fixed pointing that you determined. Continue to add exposure specifications using the same MSA configuration, and include any dispersion or cross-dispersion offsets in the MSA Configuration Editor, or in the table showing Pointings/Configurations in the Observation template.
     Custom MOS Observations using the MSA Configuration Editor
     
     
   • Specify the Pointing associated with your MSA configuration. If no pointing is specified in the Pointings/Configurations table of the MOS APT template, the pointing will be the reference position of the MSA Catalog Target used for the observation. Additionally, dithers in the dispersion and/or cross-dispersion direction can be specified in this table. Use these offsets to, for example, step the slit across the sources in the dispersion direction. Nods in the cross-dispersion direction can be added in the Nod Pattern column as well. To do so, click the Nod Pattern cell in the Configurations/Pointings table row and select one of the nod patterns from the pull-down menu.
     MOS Recommended Strategies
     
     
   • Evaluate the observation in MPT. These plans can be evaluated in the Plans tab of MPT, just like auto-generated MPT plans.
     NIRSpec MPT - Plans
     
     
   • When returning from MPT or Aladin, click the Form Editor icon to be able to see the observation template. Be sure the correct manually-designed MSA configuration is shown in the Configurations/Pointings table in the observation template.
     
     
   • Because the custom MSA configuration required the use of an MSA Catalog Target, the MOS observation will get an Assigned Aperture PA, and therefor a two-phase submission process. If needed, you may restrict the assigned angle with an Aperture PA special requirement in the proposal (refer to step 15).
     Special Requirements
     
     
   • Continue from step 11.
9. 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 randomly. However, cases that involve a handful of highly weighted sources may be impacted by the selected angle. The user should test different angles and planning strategies in any case. 
   NIRSpec MPT - Parameter Space
   
   
10. 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.  Keep your MPT plans, especially those that were used to create the observation that will be submitted in your proposal. This will aid with later updating your MOS observation for the MOS program update when the updated Catalog is available and the Aperture Position angle is assigned.
    NIRSpec MPT - Plans
    NIRSpec Multi-Object Spectroscopy APT Template
    
    
11. Return to the MOS Observation template in APT by clicking the Form Editor icon. 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 Aperture PA isn’t yet assigned. The selection of reference stars and MSATA parameters is done after the MOS observations are finalized. Leave this for the detailed program update submission. 
    
    
12. 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. Example science programs each provide a page to describe the ETC calculations for that example.
    NIRSpec Detector Recommended Strategies
    JWST ETC Target Acquisition
    Understanding Exposure Times
    NIRSpec Example Science Programs
    
    
13. In the MOS observation template, specify the correct exposure duration parameters for your science observation in your MOS observation (Readout Pattern, Groups/Integration, and Integrations).
    Understanding Exposure Times
    
    
14. 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
    
    
15. 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 and MSATA Observing Process
    NIRSpec MOS Operations - Pre-Imaging Using NIRCam
    NIRSpec Observation Visualization Tool (NOVT)
    Aperture Position Angle Special Requirements
    APT Aladin Viewer
    
    

16. A timing special requirement is needed when NIRCam pre-imaging is requested in the same program.

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

17. Add optional NIRCam or MIRI coordinated parallels, if desired. View the footprints of any 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. Note that some joint subpixel dither options will double or triple the number of NIRSpec dithers, and may require adjustment of the MOS exposure duration parameters.
    Coordinated Parallel Observations
    JWST Coordinated Parallels Custom Dithers
    APT Aladin Viewer
    
    

18. The APT Visit Planner should be run to check the scheduling of the MOS visits created by the MPT.  The APT graphical timeline may be useful for examining the layout of your MOS observation.
    APT Visit Planner
    APT Graphical Timeline
    
    

19. Your proposal is now ready for submission.
    
    

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NIRSpec MOS program update

The following roadmap should be used by those who need to submit a program update for MOS observations. The information below follows from steps outlined in the Getting Started with Planning JWST Observations article. The program update (the second phase of the MOS submission process) follows a similar approach to the proposal submission. In this phase, observers will replan their MSA observations using an assigned Aperture Position Angle (that will be populated for the user in APT) and will also select reference stars for the MOS visits. Consult the NIRSpec MOS Observing Process for a details about this process.

Roadmap for program update of MOS observations

1. Before re-planning your MOS observation, familiarize yourself with any recent updates to the MOS process, recommended strategies, or target acquisition.
   NIRSpec MOS and MSATA Observing Process
   MOS Recommended Strategies
   Background Recommended Strategies
   MSA Leakage Subtraction Recommended Strategies
   Moving Target Observing Strategies
   NIRSpec Target Acquisition
   NIRSpec MSA Target Acquisition
   
   
2. Regarding pre-imaging:
   a) If NIRCam pre-imaging was specified in your proposal, a Catalog produced by, or derived from, the NIRCam imaging pipeline can be used. The NIRCam pipeline-produced Catalog should be available in the MAST archive. Until the pipeline has been updated to include an astrometry check, however, the NIRCam data used to derive the astrometry for your catalog of reference stars and targets need to be carefully vetted against Gaia stars. In particular, the MSA Target Acquisition Procedure has a very limited ability to correct for errors in the initial roll, and so it is strongly recommended that the rotation of the reference stars on the sky be aligned with that of the Gaia frame to better than a half arc-minute in rotation.
   b) If no pre-imaging was requested in the proposal, you may have produced the source positions in your catalog from HST imaging or from imaging in past JWST programs. In either case, it is good practice to verify catalog positions against GAIA stars.  When you specify the updated MSA Catalog in the Target folder in APT, please also consider uploading the available pre-imaging data at this time for association with any pipeline produced source spectra. To do so, select Pre-Imaging availability = "Is already obtained" and you will be asked to provide the imaging for the MOS program update submission.
   MAST
   Hubble Space Telescope Finder Images and Catalogs
   Hubble Source Catalog
   NIRSpec MOS Operations - Pre-Imaging Using NIRCam
   
   
3. Whether derived from NIRCam or other instrument pre-imaging, you will need to mark suitable reference star candidates for MSATA in the Catalog. For MSA target acquisition using reference stars, four additional columns must now be added to the Catalog for the Initial Program Update submission. These columns should be labeled NRS_F110W, NRS_F140X, NRS_CLEAR, and REFERENCE. The columns starting with "NRS_" should contain the AB magnitudes of each source in each of the NIRSpec TA filters. Suitable candidates should have ABmags in the range from 18th to 25th magnitude. The REFERENCE column should have a value of TRUE, YES, or 1 when a source is a good candidate reference star (i.e., isolated, and of suitable brightness), and FALSE, NO,  or 0 otherwise. Read the NIRSpec MSATA Reference Star Selection Recommended Strategies article which describes the criteria for suitable reference star candidates and strategies for estimating their magnitudes in the NIRSpec TA filters.
   NIRSpec MOS Program Updates (a companion to this roadmap)
   NIRSpec MSA Target Acquisition
   NIRSpec MSATA Reference Star Selection Recommended Strategies
   NIRSpec MPT - Catalogs
   
   
4. Input the updated catalog as a new MSA Catalog Target into the APT Targets folder. Do not delete the old Catalog or any MPT plans or MOS observations made from it for now.  If you have updated the Catalog from new imaging (e.g. adding new sources, or deleting some), you will need to design new MPT Plans. You may also consider changing the target weights at this time.  If there were no changes to the Catalog other than the addition of columns in step 3, and its name has remained the same, you will be able to replan the MOS observations (with the same strategy as before) as described below in step 6.  
   NIRSpec MPT - Catalogs
   MPT Catalogs - Examples
   
   

   Following initial program submission, users should not change the MOS observation number.  During program implementation, MOS observations windows are identified on the long range plan and assigned an Aperture Position Angle. The internal mechanisms for keeping track of MOS observations can only happen if the observation number remains the same throughout implementation. Contact the JWST HelpDesk or your PC if you need to change the MOS observation number.

   
   
5. Examine your MOS observation(s) in APT. If an Assigned Aperture PA has been provided for your program by STScI and is indicated in the observation template, any MOS observations prepared with a different angle will be flagged as needing an update.  The assigned Aperture PA is anchored at the catalog Reference Position. Note that if there are offset pointings in the observation, a small adjustment is made to the Aperture PA of each pointing in order to keep the telescope roll fixed during an observation.  MPT automatically adjusts the Aperture PA for you when designing MSA configurations at each derived pointing.  However, when using MSA configurations created outside of MPT, and ingested into a MOS observation in APT, you will need to make sure the MSA configuration has been designed with the adjusted Aperture PA.
   NIRSpec Multi-Object Spectroscopy APT Template
   
   

6. If you are preparing your program using MPT, and have an existing MOS observation that you like, you can now simply replan it in MPT at the Assigned Aperture PA using the Replan button on the Plans pane of MPT. Note however, It is often possible to improve results by exploring different planning strategies in MPT. Investing some time in trying different strategies can result in more efficient observations with more highly-weighted sources. MPT results depend on a wide range of factors: the catalog density and extent, the source distribution, the search parameters, the number and spacing of dithers, the slit shape used, source centering constraints, etc. Changes to the observations (e.g., number of dithers and exact positions) are expected and allowed, to optimize results for the new assigned angle. In testing new Plans, first determine whether changes to your dither strategy are needed for your NIRSpec MPT-generated MOS observations. Nods and dithers are normally specified in the MPT Planner, which allows MPT to flexibly select the best pointings for those dithers. Alternatively, nods and dither offsets can be added after the fact in the MOS observation template (or even using the MSA Configuration Editor for custom MOS observations, described below).
   NIRSpec Dithers and Nods
   NIRSpec Dithering Recommended Strategies
   NIRSpec Multi-Object Spectroscopy APT Template
   NIRSpec MSA Planning Tool, MPT
   NIRSpec MPT - Catalogs
   NIRSpec MPT - Planner
   NIRSpec MPT - Plans
   
   

   OR  re-plan your custom MOS observation at the assigned Aperture PA

   designed using the MSA Configuration Editor

   • Follow the steps above in this roadmap to update and ingest the Catalog, including adding columns needed for reference star selection as described in step 3 above.
     
     
   • After an Aperture PA is assigned, observers must re-plan their custom MOS observations and submit a program update. The Assigned Aperture PA should appear in the Science Parameters section of the template. Observers will need to re-design their custom MSA configurations at the new angle. Small pointing changes are allowed at this stage.  
     
     
   • Make changes to the MSA Configuration(s) from your proposal using the updated MSA Catalog, and the Assigned Aperture PA. Follow the steps in the proposal roadmap above, in particular - the second pull-down panel of step 8 which describes the use of the MSA Configuration Editor to design a new MSA Configuration.
     JWST APT Aladin Viewer
     
     
   • Verify the parameter selections in the Exposure Specification and Configurations/Pointings tables in the Science Parameters section of the template. Check that the Pointings match the pointings at which the MSA configurations were planned. You may wish to add or remove some exposures in these tables.
     Custom MOS Observations using the MSA Configuration Editor
     MOS Recommended Strategies
     
     
   • Reference stars will need to be selected for each visit, as described below in this roadmap. Continue with the following steps in this roadmap.
     
7. When you have determined the best plan in MPT (either the updated plan from the placeholder observation, or one you have re-designed), then update the older MOS observation. To do so, as described in the NIRSpec MPT - MOS and MSATA Program Updates article, highlight the observation that you wish to replace in the APT tree. Then, select the preferred plan in the Plans pane of MPT, and click Update Observation there to apply the selected plan to the highlighted observation. The old observation will be replaced with the new one.  Keep your MPT plans, especially those that were used to update the observation. This will aid with investigating any problems you may encounter during program development.
   
   For most MOS programs, the program update must be finalized in time for the initial MOS program update submission deadline. Note that the shutter operability will evolve with time. APT will indicate when there are shutter conflicts due to evolving shutter status for planned observations. If MPT is not needed for your observation, make the changes you need in your program to use the assigned angle. Changes to your observing strategy are allowed in many cases, as long as you keep within your total allocated time. Some changes require approval, e.g. changes that could cause duplications with other programs (like changes in filters, gratings, etc.) Your program coordinator will alert you if approval is needed for a change.
   NIRSpec MOS Program Updates (a companion to this roadmap)
   MSA shutter operability
   
   
8. Using the updated Catalog source brightnesses, check the exposure and detector readout parameters for your NIRSpec MOS science observations and TA exposures 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. In the MOS observation template, update or specify the correct exposure duration parameters for your science observation in your MOS observation (Readout Pattern, Groups/Integration, and Integrations).
   Understanding Exposure Times
   
   

10. MOS observations that use an MSA Catalog Target are required to use the MSATA methodology with selected reference stars for each visit. This is so that the sources can be positioned accurately into their planned slitlets in the MSA configurations. To select reference stars for each visit, first open the Observation folder and highlight the first visit in the hierarchical tree (the left panel of the APT GUI). APT will display a view of the MSA shutters. If there are sufficient suitable reference stars designated in the catalog that are covered by the MSA footprint at the initial pointing in the visit, and are in operable shutters, there will be one or more reference star bins available for selection in the pull-down menu. Make a selection from the Reference Star Bin pull-down menu. The reference star positions should then appear in the shutter view as small black boxes. Eight candidate reference stars are nominal (5 is the minimum). Usually, the top item in the pull-down menu is the recommended selection. Selecting a bin will cause the selection of the MSATA readout pattern and filter. The default is to use the automatically-generated MSA configuration for the imaging associated with MSATA, which is meant to prevent persistence from bright objects that are in the MSA. This is suitable for most cases. This selection process must be repeated for each visit of the MOS observation to fully define the observation.
    NIRSpec MSA Target Acquisition
     

11. Add optional NIRCam or MIRI coordinated parallels, if they were requested in the accepted proposal. View the footprints of any parallels in APT in the Aladin viewer. Highlight the NIRSpec observation and click the View in Aladin button in the APT toolbar. Use the FOV button in the Aladin to see the footprints. Select single aperture if the view is too crowded.  
    Coordinated Parallel Observations
    APT Aladin Viewer
    
    
12. Select a joint dither option, if it was specified in the accepted proposal. If the NIRCam parallels are specified with joint subpixel dither options, you may need to adjust the NIRSpec exposure time as appropriate. Some joint subpixel dither options will double or triple the number of NIRSpec dithers.  
    JWST Coordinated Parallels Custom Dithers
    Coordinated Parallel Dither Tables
    
    
13. Check Confirmation image parameters if these were specified in your proposal to make sure they are suitable. 
    NIRSpec MOS Operations - Confirmation Images
    
    
14. Run the APT Visit Planner for the updated observation.  
    APT Visit Planner
    
    
15. Check that the data rate and data volume limits are not violated. Resolve any errors that may appear in the program.
    JWST Data Volume and Data Excess
    
    
16. Your program update is now ready for submission. Deadlines for submission are outlined in the NIRSpec MOS and MSATA Observing Process article.
    NIRSpec MOS and MSATA Observing Process
    

References

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

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