NIRSpec MSA Shutter Operability

Precise placement of sources into operable MSA shutters is required for NIRSpec multi-object spectroscopy (MOS) observations. One important consideration is the MSA operability—which shutters can be used and which can't— is expected to vary (to a small extent) with time. Therefore, target and background shutters must be vetted against the latest information about the MSA shutter operability status during the planning process, most importantly, just prior to the MOS program update deadline. The shutter operability status will be monitored on a regular basis during operations.

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See also: JWST Multi-Object Spectroscopy RoadmapNIRSpec MSA Planning Tool, MPT, MOS and MSATA Observing Process

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NIRSpec multi-object spectroscopy (MOS) mode uses the micro-shutter array (MSA). The MSA has nearly 250,000 micro-shutters which will be configured open or closed according to the planned MSA Configuration. There are inoperable shutters of different sorts, including failed open and failed closed shutters, vignetted shutters, and rows and columns that have been made inoperable due to electrical shorts. The shutter status information is provided in APT in a shutter operability file used by the JWST NIRSpec MSA Planning Tool (MPT) to correctly plan the observations. Additionally, one component of the operability status—the short mask information—is uploaded to the observatory to correctly command the shutters for a NIRSpec MOS observation, and to match the information in the ground system planning and processing. Finally, The Data Management System (DMS) also uses this file, as it allows for accurately calibrating and flagging the MSA data. 

Shutter operability updates in APT

See also: NIRSpec MOS and MSATA Program UpdatesMPT - Plans

The NIRSpec team will execute periodic calibration programs to track changes in the operable shutters, and in electrical shorts, which evolve with time. The exact cadence of these monitoring observations during  normal operations will be determined during commissioning, but we expect that the operability reference information will be updated on roughly a monthly basis. Updated operability files will be delivered for distribution to the different systems that use this information (including APT). For users, the latest operability information is made available after internal vetting, and is picked up by APT automatically when APT is restarted. This process does not require waiting for a new release of APT. New diagnostics (warnings) will appear on MOS exposures with planned sources that have been impacted by the shutter status changes.

Formal APT releases will occur on a less frequent basis, but whenever a new release of APT becomes available, it will include the latest MSA shutter operability information. The first big update for users will occur near the end of commissioning. It will include the new shutter operability information plus any instrument model changes determined from commissioning tests that can affect source positions at the MSA. Hence, it is expected that some planned sources will land in different shutters than when first planned, and some of these shutters may be failed shutters. If the impacts are not acceptable, the observations should be replanned in APT. All users must wait for this version of APT (or later versions) to replan MOS observations for their final MOS program updates.

How to know when a program needs replanning is described below. This article provides details about how to use updated MSA shutter operability information for replanning MOS observations prior to the MOS program update deadline (nominally 6 weeks before the start of observation execution, see MOS and MSATA Observing Process). Throughout the observing cycle, monitoring observations will likely result in shutter operability updates, but the user does not need to replan observations each time changes are indicated, precisely because the operability will continue to evolve up until observation execution. Likewise, not all impacts require replanning. 

Our best recommendation is to create the final flight-ready MOS configurations a week or two prior to your own MOS program update deadline using the latest version of APT. This can be done using the final assigned aperture position angle (Assigned Aperture PA) indicated in APT, and following the advice given in the NIRSpec MPT - Plans article.

If you prefer, you may practice the replanning process before being required to do so for the program update submission deadline. It is useful to do this with a new release of APT, for example, but please keep in mind that the MOS observation(s) will need to be re-planned closer in time to execution and prior to the final deadline.

How to check the operability status of planned shutters after an MSA shutter operability update

(1) Errors and warnings in APT

Download the latest version of APT, and load the MOS program. The new MSA shutter operability update will likely cause some impacts to a few planned open shutters in any given MSA Configuration used in a MOS observation. New warnings or diagnostics will appear on the affected exposures shown in the observation template. Hovering the mouse over the warning will pop up an explanation of which shutter, or set of shutters, has been impacted by the latest operability update (Figure 1).

Figure 1. Shutter operability warnings and diagnostics in APT

Top: The yellow triangular warning symbol on an observation in the APT tree (left menu) displays information about the impacts from a change in shutter operability status since the observation was planned. Additionally, these warning symbols also appear on individual exposures in the observation template Configurations/Pointings table, as shown. In this figure, we are showing the warnings seen from hovering over the warning symbol on a single exposure in the table (exposure #5).  
Bottom: Additional diagnostics about the impacted shutters are obtained by clicking the red Errors and Warnings icon (which looks like a red cross) in the top menu bar of the APT GUI window.  Specific shutter IDs are indicated with a "q" for quadrant, "d" for dispersion axis value, and "s" for spatial axis value.  In either view, the exposure number is shown in parentheses after the configuration ID.

(2) Exported tabular MSA Conflicts report

Additionally, APT provides the ability to export this information in tabular form. A table of all impacted shutters can be exported from APT from the File menu in the APT GUI by selecting Export, then MSAConflicts (Figures 2 and 3). The table will include an entry for all shutters (target and background shutters) impacted by a new failed closed or a new failed open shutter (See NIRSpec Micro-Shutter Assembly).

Figure 2. Export the MSA Conflicts file

The File →  Export menu in APT. Click the MSA Conflicts checkbox to obtain a table of shutter conflicts from APT. (An example is shown in Figure 3).
Figure 3. Example MSA Conflicts file

The exported MSA Conflicts file contains an entry for each shutter of a slit in the MSA Configuration that has been negatively impacted by a change in shutter operability. These can be either shutters used to observe the target, or associated background shutters in the target slit, or even master background shutters. Note that the top part of the report contains information on shutters in target slits affected by failed open shutters. It lists both the affected shutter in the target slit, and the failed open shutter causing the conflict. Unlike failed closed shutters, failed open shutters can affect planning in many nearby shutters because of the increased tendency to contaminate spectra of targets in those (normally) masked shutters surrounding the failed open. In certain cases where the user has specified that they will allow these normally masked areas to be used for planning, conflicts are expected and re-planning is not needed. The bottom part of the report indicates conflicts with failed closed shutters affecting the target or background shutters in the target slit. For both sections, specific shutter IDs are indicated with a "q" for quadrant followed by the quadrant number, "d" for dispersion axis value followed by the shutter number in dispersion, and "s" for spatial axis value followed by the shutter number in cross-dispersion.

(3) MSA Shutter View conflict indication

Lastly, you can navigate to the MSA Planning Tool Plans pane and open the "MSA Shutter View" by clicking the "Show" button on any exposure in the MPT Plans (See NIRSpec MPT - Plans). There is an MSA Configuration for each exposure in a Plan (or observation). Nodded exposures always use the same MSA Configuration at different nod positions. The conflicts between a planned source's slitlet and any new failed open or failed closed shutters are shown in magenta (Figure 4). Note that the magenta-colored conflicts are indicated for both planned Primary and Filler sources, and for a target or a background shutter. 

Planned sources in Figure 4 are shown as green dots (Primaries) and blue dots (Fillers). Each planned source is in a small slitlet, a series of adjacent vertical shutters marked in dark orange that are planned to be open to observe the sources. Most shutters in the configuration are planned to be closed. Unplanned Contaminants are black dots that happen to be in the slitlets planned for other sources. Areas in light orange are masked shutters associated with planned source slitets. They are areas that are unavailable to new source slitlets. Opening shutters on another source in those areas would contaminate the spectra of planned sources (green or blue dots). Areas in white are unused by any planned sources. They indicate available shutters for planning additional sources (e.g., MPT can be re-run to include a larger set of Primaries and Fillers, or source slitlets can be added in manually using the MSA Configuration Editor.)

Failed closed shutters are shown in dark grey. These include rows and columns masked out because of electrical shorts. Occasionally, a shutter operability update can result in new failed closed shutters in one of the planned source slitlets. The mask surrounding the conflict will be magenta-colored. (There are no examples of this in the figure.)

Failed open shutters are shown in dark red. There are very few of them in each quadrant; they are individual shutters that are stuck open, but the masked areas around them are much more likely to impact some planned source slitlets than new failed closed shutters. Two examples are marked with green circles in the figure. The failed open shutters have light red masked shutters surrounding them. MPT normally plans around these areas unless the user explicitly allows them to be used by checking a checkbox called Allow sources in areas affected by stuck open shutters in the MPT Planner. If these areas are utilized for other sources, those sources would be contaminated by the spectrum from any signal (source or background light) in the associated failed open shutter. Two planned sources in this figure (marked with red ovals) are in direct conflict with nearby failed open shutters. The conflicts are shown as a magenta mask. The failed open shutters are located inside the magenta area (look hard and you can find them!). The magenta masking can happen for 2 reasons: (1) New failed open shutters appeared because of a recent operability update, or (2) the user elected to utilize failed open shutters and masked areas around them when planning observations, and sources found in those areas of the MSA were included in the target set. In the second case, there is no impact for the user. However, when the conflicts are due to a shutter operability update (either a new failed open or closed shutter), the user must decide whether to replan or not.

The MSA Configuration Editor on the observation template will also show the shutter conflicts (See Custom MOS Observations using the MSA Configuration Editor) even if the observation was not planned in MPT. The MSA Shutter View is very similar to that in Figure 4. At the observation template, the conflicts are shown only for the first exposure of a nod set. However, the conflicts will remain the same for the other nodded exposures since the MSA Configurations of the nods remain the same. 

Figure 4. Shutter conflicts in the MSA Shutter View of MPT

An MSA Shutter View of one exposure in a MOS observation (and one MSA quadrant) showing how different areas of the MSA are being utilized in an exposure. Shutter conflicts appear in magenta. A few examples of conflicted sources are indicated with red ovals.

  • Sources: Green dot = Primary source, Blue dot = Filler source, Black dot = Contaminant
  • Dark orange shutters: Planned source slitlets
  • Light orange shutters: Areas utilized by planned slitlets.
  • Dark red shutter: Failed open shutter (within areas of light red- or magenta-colored shutters). A few failed open shutters are indicated with green circles.
  • Light red shutters: Areas affected by failed open shutters.
  • Magenta: Shutter conflicts between planned sources and a failed open or failed closed shutter.
  • Dark grey shutters: Failed closed shutter or shorted rows or columns of shutters that have been masked.

How to fix an MSA Configuration to correct shutter conflicts with new inoperable shutters

There are 2 different ways to fix an MSA Configuration to avoid shutter conflicts with new inoperable shutters:

  • The best solution is to download the latest version of APT, load the program, navigate to MPT, and re-plan using MPT:
    • Either create a new MOS Plan in MPT from scratch using the Planner, OR use the “Replan” button on the Plans pane to apply an existing observation plan strategy to make a new observation Plan. Then update the existing observation using the new Plan using the “Update Obs” button on the Plans pane, as described in the Plans article.
  • Edit the MSA Configuration manually using the MSA Configuration Editor in the observation template to omit any conflicting sources by closing the slitlet shutters they are in.  (This approach is not recommended, especially if there are dithered observations, as it is a tedious workflow).

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