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

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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.

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 (**Aperture PA,** θ) is shown as measured on the sky.

Two-step submission process and timeline

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

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
A fully-defined and executable program update submission that follows an STScI angle assignment. This process will have 2 deadlines for initial and final submission stages. The general MOS process and Cycle observing timelines are described in this article.

All NIRSpec MOS programs will have 2 submission stages: an initial proposal submission and a program update submission stage for the flight executable MOS observations. The MOS proposal deadline is the same as that for all other JWST proposals. The additional program update submission will have 2 associated deadlines that 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 MSA Catalog (i.e., with or without NIRCam pre-imaging defined in the same proposal).

(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 MSA 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 duration 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. Therefore, 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 Aperture PA will be communicated back to teams and will appear in the MOS observation APT 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 Aperture PA. At this point, the target acquisition reference stars should be defined and submitted if the MSATA option is used for the observations. There are 2 deadlines associated with the program update submission:

1) MOS Initial Deadline - two months to observation window: A fully executable MOS program should be submitted for review by the instrument scientists and program coordinators. The full program should be defined and the MPT used to create the MOS observations. It is at this point that 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.

2) MOS Final Program Deadline - one month to observation window: After interaction and iteration with both NIRSpec instrument scientists and program coordinators, the final end-to-end executable MOS program submission should be made one month prior to the observation window. This final program deadline allows users to verify their initial submissions and make use of the latest shutter operability information, which evolves with time.

It should 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 well before your final 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 final deadline will be accepted. If time is too short (i.e., within 30 days from your observation window start) you may request that STScI pull your observations for re-scheduling at a later time. Failure to comply with this 2-deadline program submission timeline can cause delays in observation verification that might cause an observing window to be missed and a MOS observation replan to be necessary.

This set of program update submissions can occur only after any NIRCam pre-imaging that was proposed has been obtained. (Iteration with the instrument scientist and program coordinator will be necessary for programs that request fast (<60 days days) pre-imaging turn-around).

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

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 initial submission in the program update process, suitable reference stars must be present in the user's input MSA 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.

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 Position Angle special requirements that are science-driven.

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

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).
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.
The fully defined and executable initial NIRSpec MSA program submission initial 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 preaging 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 submission deadline 1 month prior to the observation plan window.
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

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 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 be in conflict with the initial submission date for the program review (which is 2 months in advance of the start of the plan window; see Figure 2). 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 NIRSpec instrument scientists and program coordinators to quickly prepare executable MOS observations as there is very little margin for review and scheduling.

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 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.

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.

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.

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	30 Nov 2022 Updated MOS process deadlines 01 Apr 2021 Added multi-excerpt Figure 1 and re-organized for clarity. 09 Oct 2020 MOS process and deadline updates 25 Mar 2018 Clarification of MOS process and highlighted box on text regarding ON HOLD requirement
Originally published	27 Sep 2017

JWST User Documentation

NIRSpec MOS and MSATA Observing Process