NIRSpec MOS Proposal Checklist

The checklist for proposing for JWST NIRSpec MOS observations is outlined.


As stated in the JWST NIRSpec Multi-Object Spectroscopy APT Template Guide, the NIRSpec MOS science observations are not filled out in the APT template in the same way as all other observing modes.  This is because there are better, more automated ways to search for optimal plans for MOS observations to maximize the acquired science spectra.  In order to clarify the MOS planning process, a step-by-step checklist for proposing NIRSpec MOS science is summarized below for users.  Note that this NIRSpec MOS science mode proposal checklist will make the best sense if used in conjunction with the APT and the NIRSpec MSA Planning Tool (MPT).

NIRSpec MOS Proposal Checklist 

  1. Users should first determine the range of feasible MOS observation Aperture Position Angles using the General Target Visibility Tool (GTVT). The NIRSpec Observation Visualization Tool (NOVT) or the Aladin viewer in APT may be used to quickly discern whether any feasible position angles will not work for the science, and may warrant a special requirement to restrict the orientation of the proposed observations.
  2. NIRSpec MOS mode users typically do not directly fill out the APT template as for other modes (except for Moving Targets and some other exceptional cases).  Users typically need to run the 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 derived after an angle has been assigned. Using the MPT also helps to predict the number of targets to expect for each pointing for a given observing strategy.  For MPT to be able to work, the user must be connected to the internet so MPT can check whether an Aperture Position Angle (APA) is feasible.
  3. NIRSpec observations planned with MPT require planning Catalogs of science sources.  Observers should create a catalog of sources that covers an area larger than the MSA field of view. For one, this will help when selecting suitable reference stars for MSATA target acq. The catalog should be as complete as possible to help the user recognize when source spectra may become contaminatedThe accuracy of spectroscopy (and target acquisition) directly depends on the Catalog's relative astrometric accuracy.
  4. Observers should determine whether existing imaging of their fields of interest will provide the required accuracy for MSATA target acquisition and science.  If existing imaging is inadequate, the observer should propose for NIRCam pre-imaging in the same cycle in their proposal. NIRCam 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 followup at any potential APA
  5. Proposals that request NIRCam pre-imaging to plan NIRSpec MSA observations (MOS science or MSATA) should have a TIMING APT special requirement (specifically, an AFTER Observation Link) on the NIRSpec observation separating that observation from the NIRCam pre-imaging observation(s) by a minimum recommended 60 days. The NIRSpec Observation Visualization Tool (NOVT) can be used to visualize and help plan the pre-imaging observations relative to the NIRSpec observation.
  6. The MPT should be run at several available APA to to more accurately estimate 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 isotropically in angle. However, science cases that involve a small number of highly weighted sources may be impacted by the selected angle. The user should test this in any case).
  7. Once users have an MPT plan they are happy with, they click Create Observation from the "Plans" pane in MPT to populate the NIRSpec MOS Mode Template with pointings, MSA configurations and exposure parameters. When MPT is used to create observations, an “Auto_Targetis created and added to the APT Targets folder.
  8. The user should continue to fill out the remaining elements of the MOS APT Template, including the decision of whether or not to add Confirmation Images. Confirmation images provide a means to measure the observed positions of science sources in shutters after program execution.
  9. The NIRSpec MOS observation should ideally not have a specific APA special requirement added. However, some use cases may need such constraints. For schedulability, a minimum range of ~20 to 30 degrees is recommended.  Observation planning for MOS science in the MPT will use an APA, however this angle is not used outside of the MPT
  10. Target Acquisition (TA) in the proposal:  Standard Target Acquisition (MSATA) reference alignment targets, called reference stars, and related TA parameters for the science observations cannot be defined at the proposal submission time because the execution Aperture Position Angle isn’t yet assigned. Verification exposure parameters for the VERIFY_ONLY TA option can be defined in the proposal at the Observation level. Verification images are used to verify the pointing. This image is taken at the end of the visit (at the final pointing only) with the MSA in the ALLOPEN configuration.
  11. If you have a use case that requires mosaicking, you should create it with the Manual Planner and specify the mosaic at the Observation level in APT. (MOS observations created by the MPT have dithers that have been optimized for the scene and should not also have mosaics.)
  12. The APT Visit Planner should be run to check the scheduling of the MOS visits created by the MPT.

Related links

JWST User Documentation Home

NIRSpec Multi-Object Spectroscopy (MOS) Mode

NIRSpec MOS Observing Process

NIRSpec MSA Planning Tool, MPT

APT Visit Planner

JWST APT Aladin Viewer

APT Smart Accounting


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

Last updated

Updated March 26, 2018

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Published June 2, 2017