NIRSpec MOS Observing Process

The JWST NIRSpec multi-object spectroscopy (MOS) observing mode requires a multi-step process for submission and update of proposals to create executable visits. This applies to all MOS spectroscopy (with or without NIRCam pre-imaging), as well as IFU and FS observations that use the MSA-based NIRSpec target acquisition process (MSATA).  

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See Also: NIRSpec MOS Proposal Checklist (internal)

The NIRSpec multi-object spectroscopy (MOS) mode provides the means to 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 instrument Aperture Position Angle (APA)1 . This is required in order for the science sources to align properly with the very small 0.2" × 0.46" MSA shutters. For JWST, restrictions in the APA translate to constraints for the execution time window. Therefore, to permit more flexible scheduling of observations and to ensure correct execution angles, the NIRSpec MOS observing process will consist of two steps.

NIRSpec MOS observations that require a precise APA will be planned in two steps:  (1) proposal preparation and submission and (2) observation update for flight executable MOS visits after an APA is assigned by STScI. During the first step, placeholder MOS observations must be included in order to quantify overheads and multiplexing efficiency. This will typically require using the MSA Planning Tool to generate test configurations at arbitrary (but feasible) APA.

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 also follow this multi-step process. This article describes the observing process and timeline, and how NIRSpec APA constraints can affect observation windows.

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The NIRSpec MOS and MSATA observing process

See also: NIRSpec MOS Proposal Checklist (internal)NIRSpec Target AcquisitionNIRSpec Target Acquisition Recommended Strategies, Target Visibility Tools, and NIRSpec MSA Planning Tool, MPT

JWST has significant pointing constraints on target visibility windows resulting in limited availability of orientations for a given pointing. Therefore, a scheduling team at STScI will assign a fixed APA to each MOS science program and other programs using MSATAAt proposal submission, NIRSpec MSA-based observations will not be ready to execute on the telescope. Updates to accepted programs will be needed to create more detailed observations that are flight-ready. Users should submit a NIRSpec MOS proposal prepared using the Astronomer's Proposal Tool (APT) following the instructions and guidelines for using the MSA Planning Tool (MPT). The MPT should be used to prepare placeholder visits for the requested observing time. MPT will use a placeholder APA  that will not constrain the scheduling of accepted programs but will be used to estimate overheads.  Only observatory special requirements on position angle, if present, will constrain the APA and scheduling.

Accepted NIRSpec MOS or MSATA proposals will be assigned an APA after the JWST cycle Long Range Plan (LRP) is created. The assigned angle will correspond to an execution window that has been analyzed for schedulability compared to all accepted JWST science programs. This assigned execution NIRSpec APA will be communicated back to teams so they may start the MOS observation planning process. The on-sky executable MOS science or MSATA observations are planned to their assigned APA by users, after programs are accepted by the TAC. “Flight ready update” MSA programs are constructed and submitted by observing teams through APT, with the same interface as the proposal. Figure 1 shows a timeline of this NIRSpec MOS or MSATA proposal submission process in the case where source planning catalogs exist and no pre-imaging observations using NIRCam are necessary.

Some science use cases may require constraints on APA to improve multiplexing. MOS or MSATA proposals that request constraints on the NIRSpec APA are discouraged, but not prohibited if the science justifies it. Some considerations regarding APA for MOS science cases are presented below.

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 will 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.

Figure 1. Timeline of the NIRSpec MOS observing process without NIRCam Pre-Imaging

A timeline figure of the JWST NIRSpec MOS observing flow during an observing cycle.


Deadline for Submission of MOS Flight-Ready Program Updates

Program updates for NIRSpec MOS proposals, as well as programs using MSATA, will be due to STScI 28 days (four weeks) prior to the observation scheduling window. Table 1 shows the reasoning behind this deadline, outlining  the expected timeline for STScI review.  Users are welcome to submit programs far in advance of this 28 day window, but should also be advised that the shutter operability can change on short timeframes, so final planning of MOS programs should be done close to the deadline to prevent the loss of primary science targets. It is anticipated that 28 days is the minimum time needed by STScI staff to process MOS or MSATA flight ready program updatesThis process will be re-assessed during Cycle 1 science operations. The timescale for reviews of the MOS and MSATA flight updates may be adjusted once experience is gained.


Table 1. STScI review of flight executable MOS or MSATA observations

Time frame before science observation windowSTScI NIRSpec flight executable program review activity
28 to 21 DaysInstrument scientist review of MOS or MSATA observations
21 to 14 DaysProgram coordinator checks of MOS or MSATA observations
14 to 0 DaysShort term scheduling of executable visits



Example effect of fixed Aperture Position Angle constraints on observation execution windows 

Requesting fixed or a highly constrained NIRSpec APA for MOS science or MSATA will translate directly to constraints on schedulability in the observing Cycle. Figure 2 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 2 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 forbidden.

  • 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 2. 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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