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. 

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Factors influencing the MOS and MSATA process decisions

See also: MOS Roadmap, JWST Position Angles, Ranges, and OffsetsJWST Target Visibility ToolsNIRSpec MSA Planning Tool, MPT and Pre-Imaging Using NIRCam

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 (APA)* (see Figure 1). This is required in order for the science sources to align properly with the very small 0.2" × 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.

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.

Figure 1. Aperture Position Angle

A given NIRSpec MSA observation must be executed at a specific telescope orientation angle so that the selected sources will fall into the open shutters as planned. Because of the telescope's orbital position and sunshield constraints, a given orientation may only be possible for limited periods during the year or not available at all. The JWST Target Visibility Tools can be used to determine the range of feasible orientation angles for a target throughout the telescope's orbital cycle.

For most NIRSpec MOS observations, it is recommended to experiment with several different feasible Aperture Position Angles (APA) prior to initial proposal submission. The "Planned APA" was that used during planning. Once a proposal is accepted, it will be assigned a fixed orientation (an "Assigned APA") and a corresponding window of time when observations will be made. 

For proposal planning and submission, use any feasible APA from the orientation angles provided by the JWST Target Visibility Tool for the time of observation. For final program submission (containing flight-ready program updates), use the fixed orientation Assigned APA that has been assigned to your observation by the schedulers at STScI. This assigned angle will correspond to one or more plan windows for execution in the scheduling timeline.

The APA (an angle between 0 and 360 degrees) specifies the orientation of the cross-dispersion axis of the MSA aperture measured East of North. Figure 2 shows how this angle is measured on the sky. The "Y" axis represents the cross-dispersion direction of the MSA.

Figure 2. Aperture position angle

The black arrow represents the North direction. The blue arrows represent the direction of the Ideal X and Y axes. Dispersion is along the "X" axis. The Aperture Position Angle (θ) is shown as measured in the sky.
Programs that have an orientation or timing special requirement added to the observation can restrict scheduling opportunities. Most NIRSpec MOS programs should be submitted without added orientation restrictions, in order to allow the schedulers to find suitable windows for them.



Two-step submission process and timeline

STScI will assign the orientation angle after proposal acceptance. The MOS observing process accordingly calls for 2 submissions:

  1. an initial "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 STScI angle assignment. The general MOS process and cycle 1 timelines are described in this article.

All NIRSpec MOS programs will have 2 submission stages: an initial "proposal submission" and a "program update" submission with flight executable MOS observations. 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. 

Figure 2 shows a timeline of this NIRSpec MOS proposal submission process. This process applies to NIRSpec MOS with or without a final catalog (i.e., with or without NIRCam pre-imaging defined in the same proposal). 

Figure 2. Timeline of the NIRSpec MOS observing process for cycle 1

(1) Initial proposal submission

Users should submit a NIRSpec MOS proposal prepared with the MSA Planning Tool (MPT). The MPT will create placeholder visits for the requested observing time using any user-selected feasible APA for the target area of interest. In APT, this is recorded as the Planned Aperture PA. If pre-imaging is needed to derive a final catalog for the MOS observations, observers should run MPT with a simulated catalog of similar extent and source density as the final catalog that is expected to result from the pre-imaging. All MOS observers should experiment with MPT to determine the best observation strategy for their 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 goals, and to estimate the multiplexing efficiency for the observing team and for the TAC assessment. After TAC acceptance, the placeholder visits will also be used to estimate visit durations for the desired pointings on the sky to facilitate planning and scheduling. For MOS proposals with NIRCam pre-imaging included, there are some additional considerations and requirements for the initial proposal submission. 

For JWST, restrictions on the APA translate to constraints on the execution time window. Therefor, any added special requirements on position angle 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.

(2) Program update submission

After proposal acceptance, the JWST cycle Long Range Plan (LRP) is created, whereby the schedulability of pending observations is examined and corresponding observing windows are planned. 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 APA will be communicated back to teams and will appear in the MOS Observation template so teams may continue the MOS observation planning process. Observers of accepted programs will re-plan MOS observations with the MPT using the Assigned APA. If the observing strategy has not changed, this can be accomplished easily and in one step in the MPT.

For any NIRSpec visit using MSATA, including Fixed Slit and/or IFU observations, this angle assignment process will be similar.

Additionally, in this second submission stage, the observing team will further define target acquisition parameters and select reference stars from their source catalog for each visit using MSATA, so that the visits become flight-executable. Reference stars are selected for the first science position of each visit. If suitable reference stars in the field have been identified in the MSA catalog, this has also been designed to be a simple singe selection step in APT.

Please note - 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 program update, suitable reference stars must be present in the user's input Catalog in order for this process to work. A detailed description can be found in the article on MSATA, and in the article NIRSpec MSATA Reference Star Selection Recommended Strategies.



Deadlines for submission of MOS flight-ready program updates in cycle 1

In cycle 1, plan windows will be inserted into the LRP, and the assignment of the APA for each observation will occur well in advance of the cycle start. The refined astrometric solution, however, will not be available until late in Commissioning, as shown in Figure 2. Both elements are needed for preparing the program update with flight-ready MOS observations. In order to be able to execute MOS observations early in cycle 1, we have defined an early deadline for program updates that applies only to the subset of accepted MOS programs with early plan windows, as described below. 

EARLY deadline

If the LRP plan windows have been scheduled to start within the first 3 months of science operations, there will be a fixed deadline to facilitate fast turn-around internal review and processing, so that MOS observations can begin as early as possible in the cycle. The deadline is expected to be about 3 weeks after the final astrometric solution becomes available in APT, within a few weeks of the start of science operations. This deadline is indicated in Figure 2 as the first of two program update deadlines (big red arrows). The date of this deadline will only be known for certain when the final astrometric solution is available. It will be communicated in a timely manner to the PIs of those MOS proposals that are scheduled to execute in the first 3 months. For this subset of MOS programs, STScI will work closely with the PIs of accepted programs to ensure these programs are scheduled in time. MOS programs that have existing catalogs with sufficiently accurate source coordinates and therefore do not require pre-imaging are good candidates for this initial period of cycle 1.

NOMINAL deadline

If none of the MOS observation plan windows start in that initial 3 month period of the cycle, then the program update deadline will instead be 6 weeks in advance of the plan window start. This deadline applies to the remainder of the MOS programs in cycle 1, including most MOS programs with pre-imaging. This is shown as the second of the two large red arrows in Figure 2.

The deadline for Program Updates for MOS proposals will depend on when their MOS observation plan windows start. Figure 2 indicates where the deadlines occur for the two subsets of MOS programs. It is estimated that 42 days (6 weeks) is the minimum time needed by STScI staff to process MOS flight ready program updates. Following program update submission, MOS programs will be reviewed by instrument scientists, checked by the program coordinators, and put on the short term schedule prior to execution. Deadlines for the program updates have taken this review and scheduling period into account.

It should also be noted that the MSA shutter operability can change on short timeframes. APT will alert the observer of any new failed shutters that affect planned sources when your program is loaded into APT. Hence, if you submitted your program update before the deadline and your science is significantly impacted by shutter operability changes after submission, you may wish to update your program again. Program updates up to the deadline will be accepted. If time is too short (i.e. within 42 days from your observation window start) you may request that STScI pull your observations for re-scheduling at a later time.



MOS observations with pre-imaging

In the initial 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 Aperture 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 6 weeks before the start of the NIRSpec MOS spectroscopy window to allow for enough time for internal review and scheduling.
  4. Finally, the NIRSpec MSA science or TA visits will be scheduled at the prescribed fixed APA used to plan the observation.

Separating the pre-imaging and MOS observation

In cycle 1 especially, 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 catalog, and submit the program update in time for internal checking and processing. 

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, AFTER BY 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 (6 weeks), but 60 days is recommended.

Timing recommendations:

  • Users may view the visibility windows using the General Target Visibility Tool (GTVT). If the target has two visibility windows per cycle, then it is recommended for the observing teams to add an "AFTER BY" 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 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, an "AFTER BY" 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.

  • The scheduling system will additionally impose a final firm deadline for program update submission that is 6 weeks 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 (6 weeks). Note that this creates a conflict with the latest submission date for the program update (which is also 6 weeks in advance of the start of the plan window). If the shortened timeframe between observations is scientifically justified, allowances will be made to process these programs quickly, but observers will need to work closely with their program coordinators to quickly prepare executable MOS observations as there is very little margin for review and scheduling.

This process and timeline will be re-assessed during cycle 1 science operations. The timescale for reviews of the MOS program updates may be adjusted once experience is gained.



FS and IFU observations using MSATA

All JWST NIRSpec IFU and FS observations that use the MSA-based target acquisition method (MSATA) must also execute at a fixed APA and will follow a similar multi-step process to what has been described above.  If pre-imaging is needed to define reference stars for MSATA, the science observation will need to have similar AFTER BY special requirements to separate the two 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 APA 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

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 two 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 eight 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 eight days long—it is defined by an APA 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.



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Latest updates
  •  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