NIRSpec MOS and MSATA Observing Process

Precise placement of sources into MSA shutters is required for NIRSpec multi-object spectroscopy (MOS) observations. This can only be done at a defined orientation angle and pointing. All MOS observations will need an angle assignment (except for MOS observations configured into a long slit). Additionally, any NIRSpec program, including IFU and FS mode observations, that uses MSATA with a selection of reference stars will also follow this process. 

On this page

Factors influencing the MOS and MSATA process decisions

See also: JWST Multi-Object Spectroscopy Roadmap, JWST Position Angles, Ranges, and OffsetsJWST Target Visibility ToolsNIRSpec MSA Planning Tool, MPTPre-Imaging Using NIRCam, MSA shutter operability

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The NIRSpec multi-object spectroscopy (MOS) mode can simultaneously obtain spectra of multiple objects within the 3.6' × 3.4' field of view of the micro-shutter assembly (MSA). In order to accurately open the correct micro-shutters on the science sources of interest, NIRSpec MOS mode exposures must be planned and executed at a precisely-specified MSA aperture position angle (designated Aperture PA in APT, and abbreviated as "APA" throughout JDox) (see Figure 1). This is required in order for the science sources to align properly with the very small 0.20" × 0.46" MSA shutters. 

JWST has significant pointing constraints on target visibility windows resulting in limited availability of orientations for a given pointing. To improve scheduling flexibility, a multi-step process which includes the assignment of the APA following program acceptance has been defined for these programs. MOS observations can only be finalized after the APA has been assigned.

Figure 1. Aperture position angle

MOS submission process and timeline

See also: Resources for MOS and MSATA Program UpdatesNIRSpec MPT - CatalogsNIRSpec MSA Shutter Operability

STScI will assign the orientation angle after proposal acceptance. The MOS observing process accordingly calls for two submissions:

  1. proposal submission that will provide pointings, observation links, and visit durations for the time allocation committee (TAC) and the long range plan (LRP), and
  2. A fully-defined and executable Program Update submission that follows an STScI angle assignment. The general MOS process and Cycle observing timelines are described in this article.

All NIRSpec MOS programs have 2 submission stages: The MOS proposal deadline is the same as that for all other JWST proposals. The additional Program Update submission deadline will depend on when the program's MOS observation Plan Windows begin. For any NIRSpec visit using MSATA, including fixed slit and/or IFU observations, this angle assignment and two-stage submission process will be similar.

Figure 2 shows a timeline of this NIRSpec MOS proposal submission process for Cycle 2 and beyond. This process applies to NIRSpec MOS with or without NIRCam pre-imaging defined in the same proposal. 

Figure 2. Timeline of the NIRSpec MOS observing process for an observing cycle

(1) Proposal submission

At proposal submission, users should submit a NIRSpec MOS proposal prepared with the MSA Planning Tool (MPT), or directly in the MOS observation template, including MSA configurations. MPT will create placeholder visits for the requested observing time using any feasible user-selected APA for the target area of interest. In APT, this is called the Planned Aperture PA. If pre-imaging is needed to derive a final MSA Catalog for the MOS observations, observers are recommended to generate observations using MPT with a simulated Catalog of similar extent and source density as the final Catalog that is expected to result from the pre-imaging. MPT can be used to determine the best observation strategy for the science (e.g., number and size of dithers, slitlet length and source centering constraints, etc.). The placeholder visits are used to determine the total time required to carry out the science program, and to assess the multiplexing efficiency for the observing team and for the TAC assessment. After TAC acceptance, the placeholder visits facilitate planning and scheduling using the estimated visit durations and desired pointings on the sky. For MOS proposals with NIRCam pre-imaging included, there are some additional considerations and requirements for the proposal submission. 

For JWST, restrictions on the APA translate to constraints on the execution time window. Therefore, any added timing or position angle special requirements will constrain the scheduling, and must be justified in the proposal. An example is shown below in Figure 3. Proposals that request constraints on the NIRSpec APA are discouraged, but not prohibited if the science justifies it. Some considerations regarding APA constraints for MOS science cases are presented below.

Please note that the selection of TA parameters for MSATA is not required at proposal submission. During program update, after an APA has been assigned by STScI, the final target acquisition parameters must be defined by the user through the selection of a set of reference stars for each visit. At the MOS Program Update deadline described below, suitable reference stars must be present in the user's input MSA Catalog in order for this process to work. More details can be found in the article on MSATA, and in the article NIRSpec MSATA Reference Star Selection Recommended Strategies.

(2) Program Update submission

After proposal acceptance, the JWST cycle long range plan (LRP) is created. During this process, the schedulability of pending observations is examined and corresponding Plan Windows are selected. A scheduling team at STScI will then assign a fixed APA to each MOS science observation (and other observations using the MSA target acquisition methodology, MSATA). This Assigned Aperture PA will be emailed to the program PI and will appear in the MOS observation APT template. Observers of accepted programs will re-plan MOS observations in APT using the Assigned Aperture PA

The MOS Program Update submission deadline for Cycle 2 and beyond is shown in Figure 2 and described below with other MOS process milestones.

MOS Program Update deadline

  • When: Two months prior to the start of the Plan Window. 

A fully-executable MOS Program Update submission is required of all MOS programs at this time. This program update should include the final Catalog, target acquisition parameters, and selected reference stars for each visit using MSATA, so that the visits are flight executable, or as close to that as possible. Following submission, the program coordinators will ingest and verify the programs, and the NIRSpec instrument scientist will review the MOS observations and alert the PI to any problems with the implementation.  

Failure to meet the MOS Program Update submission deadline could result in a missed Plan Window, or even a missed visibility window. If that happens, since the APA is tied to the Plan Window, a MOS observation replan will be required.

MOS program finalized

  • When: One month prior to the start of the Plan Window.  

In all cases, MOS programs must be finalized one month prior to the plan window start.  Issues raised in the initial Program Update submission must be addressed in a final program update, one month prior to the plan window start. Likewise, observations scheduled early in the Cycle or with late pre-imaging that miss the initial Program Update submission timeframe will need to be finalized one month before the start of the plan window.

Assigned Aperture PA change requests

There are cases when the science would benefit from a different Assigned Aperture PA. STScI can accommodate requests for changes to the APA within the parameters outlined below.

There are cases where the science would benefit from a different Assigned Aperture Position Angle (Assigned APA), or from splitting the MOS observation into two or more observations.  

Assigned APA change requests that do not require TTRB approval:

  • Must be made in advance of the MOS Program Update submission deadline,
  • Cannot alter the assigned Plan Window  (Note that small APA changes of less than a degree are less likely to result in a Plan Window disruption.),
  • Cannot increase the allocated time

Further, the finalized flight-ready program must be submitted by the MOS Program Finalized date (one month in advance of the Plan Window start), and preferably by the MOS Program Update submission deadline (two months in advance of the Plan Window start).

APA changes like these necessitate a re-plan of the observation and technical review of the new MSA configurations. Thus, only one such change will be allowed per MOS observation (either a MOS observation split, or a change to the APA) and the request should be made as early as possible.  APA change requests (or requests to split a MOS observation) made outside the limitations outlined above must go to the TTRB and will require strong science justification for approval. This could potentially result in the need to push the observation into a future visibility period.

MOS observations with pre-imaging

        See Also:  NIRSpec MOS Operations - Pre-Imaging Using NIRCam

In the proposal submission, the observing program must include:

  • the flight-ready NIRCam pre-imaging observations fully specified using the NIRCam imaging template (see Pre-Imaging Using NIRCam for additional information);
  • placeholder visits for NIRSpec MSA-based spectroscopy that request the appropriate amount of time for the MOS science;
  • any observation links or special requirement constraints requested on the final program visits—these observation links include any Timing or Position Angle special requirements that are science-driven.

The timeline for MOS observing with pre-imaging includes a number of milestones:

  1. After the TAC meets and programs are approved, the long range planning team at STScI will incorporate accepted programs into an LRP for the cycle. The schedulers will create Plan Windows for the NIRCam pre-imaging and MOS observations based on the target visibility, visit durations and observation links, and will assign a fixed APA value to the MOS observation(s). 

  2. After the NIRCam visits execute and the pre-imaging is acquired, the pipeline-generated mosaics and catalogs will be uploaded to the MAST archive. The planning process for the NIRSpec spectroscopic science and target acquisition can commence once the images and Catalogs are ready.

  3. The fully defined and executable NIRSpec MSA program submission due date will be 2 months before the start of the NIRSpec MOS spectroscopy window (as seen in Figure 2) to allow for enough time for internal review and scheduling.  If the NIRCam pre-imaging observations include a fast turn around less than this 2 month submission window, teams will need to work with their NIRSpec instrument scientist and program coordinator to optimize the timing and program definition to ensure the MOS program can be submitted in time for the Final MOS program submission deadline 1 month prior to the observation Plan Window. 

  4. Finally, the NIRSpec MSA science or TA visits will be carried out at the prescribed fixed APA used to plan the observation.

Separating the pre-imaging and MOS observation

See Also: Timing special requirements, JWST General Target Visibility Tool Help

Proposals that request NIRCam pre-imaging to plan NIRSpec MSA observations (MOS science or MSATA) should also be submitted with an observation Timing special requirement (specifically, an AFTER Observation Link). The special requirement should be added to the NIRSpec observation, linking the NIRCam imaging observation(s) and the NIRSpec spectroscopy (e.g., AFTER <NIRCam observation> by 60 days). The absolute minimum separation between the NIRCam pre-imaging and NIRSpec observation is 42 days, but 60 days is recommended.

Observers should separate their NIRCam pre-imaging observations from their NIRSpec spectroscopy observations in the initial proposal submission. For MOS programs that include NIRCam pre-imaging, the pre-imaging and MOS observations must be separated by a minimum period of time that is needed for the observer to prepare the final MSA Catalog, and submit the program update in time for internal checking and processing. 

Timing recommendations:

  • Users may view the visibility windows using the General Target Visibility Tool (GTVT). If the target has 2 visibility windows per cycle, then it is recommended for the observing teams to add an "AFTER Observation Link" timing special requirement on the NIRSpec observation that would place the two observations into separate visibility windows. This approach would allow enough time for the analysis or correction of the NIRCam astrometry prior to the NIRSpec MOS Program Update deadline.

  • When that is not possible (e.g., when there is only one visibility window for the target) or the science goals prevent it, the timing special requirement is still needed. The recommended 60 days is intended to provide sufficient time for making final plan updates and for post-submission internal checking.

  • There will be a final firm deadline for program update submission that is one month in advance of the start of the NIRSpec spectroscopy scheduling window. 

The absolute minimum allowed time frame between JWST NIRCam pre-imaging observations and the JWST NIRSpec spectroscopy observations is 42 days. Note that this may conflict with the MOS Program Update submission deadline (which is 2 months in advance of the start of the Plan Window; see Figure 2). If the shortened timeframe between pre-imaging and MOS observations is scientifically justified, allowances will be made to process these programs quickly.  For this to happen, observers will need to work closely with their NIRSpec instrument scientists and program coordinators to  prepare executable MOS observations by the "MOS Program finalized" date.

FS and IFU observations using MSATA

See Also: NIRSpec MSA Target AcquisitionResources for MOS and MSATA Program Updates

The MSATA method is designed for MOS observations, but can be used for FS and IFU observations as well.  However, in practice, the NIRSpec wide aperture target acquisition (WATA) has proven to be extremely accurate at positioning science targets in the NIRSpec apertures. Use of the MSATA method for FS and IFU must be well justified in a proposal since it is more timing consuming, requires a fixed APA and catalog definition of TA reference stars, and in many cases it may not deliver more accurate TA performance compared to the WATA method.

All JWST NIRSpec IFU and FS observations that use the MSA-based target acquisition method (MSATA) must execute at a fixed APA and will follow a similar multi-step process to what has been described above for MOS observations. If pre-imaging is needed to define reference stars for MSATA, the science observation will need to have similar AFTER Observation Link timing special requirements to separate the 2 observations, making it less likely these programs will be scheduled in the first 3 months of the cycle. Except for the TA parameters, the science (FS or IFU) observations should be fully specified at initial proposal submission. Any angular constraints (fixed, or a range of angles) should be specified and justified in the proposal. An Assigned Aperture PA will be communicated to the PI, even in the case that the observer has requested a range of angles, or a fixed angle in their proposal. Depending on when the observations are scheduled, the deadline for submission of the program update containing the full TA details will follow a similar schedule to that shown in Figure 2.

Effect of aperture position angle constraints on schedulability

See Also: APT Visit Planner

Requesting a fixed or a highly constrained NIRSpec APA for MOS science or MSATA will translate directly to constraints on schedulability in the observing cycle. Figure 3 shows 2 APT Visit Planner views of available execution windows for observations planned with identical MOS planning parameters and science duration. Both observations are at the same pointing position; the feasible APA span a range of 30°. The top visit planner view in Figure 3 is for an observation with an APA special requirement that has a range of 0° (i.e., a fixed APA). This observation has only 8 days of schedulability over a year-long JWST observing cycle. The bottom Visit Planner view has no APA special requirement, and an available execution window of 54 days. Very strict constraints on execution position angles are discouraged but not prohibited, if science justification is given.

  • For optimal schedulability of NIRSpec MOS and MSATA observations, programs should have no APA special requirements.  
  • An available angle constraint range of around 20°–30° is reasonable,  but all requests for tighter APA constraints on MOS or MSATA programs must be well justified in the technical case for the proposal.

Figure 3. Visit Planner observing windows for two planned observations at constrained vs. unconstrained APA

Shown here are visit planner observing window views of a highly constrained proposed APA (top) and unconstrained proposed APA (bottom). The more constrained observing window is only 8 days long—it is defined by an Aperture Position Angle special requirement with a 0° range. The unconstrained position angle plan has no special requirement constraint over the possible NIRSpec position angle range of 30°. This allows for science to be scheduled more optimally in the long range plan (LRP), over a wider 54 day window.

Latest updates
    APA change requests reworded
    Additional updated description of deadlines

    Updated description of deadlines

    Updated MOS process deadlines 

    Added multi-excerpt Figure 1 and re-organized for clarity.

  •  MOS process and deadline updates

    Clarification of MOS process and highlighted box on text regarding ON HOLD requirement
Originally published