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.

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

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: MPT - 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 is roughly monthly, but STScI expects that the operability reference information will be updated less frequently, and as needed. 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 it 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 occurred near the end of commissioning. It included new shutter operability information and instrument model changes determined from commissioning tests that affected source positions at the MSA. There have been several operability updates during Cycles 1 and 2. These can be expected to continue to occur infrequently.  Also, further model updates are expected to be infrequent, if needed at all. With a model update, some planned sources may land in different shutters than when first planned, and with an operability update, some of these shutters may be failed shutters. If the impacts are not acceptable, the observations should be replanned in APT. It is important to download and use the latest version of APT to replan MOS observations, especially for the final MOS program updates.

This article provides details about how to know when a program needs replanning after a model or shutter operability update. It also describes 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, shutter operability updates may occur, but the user does not need to replan observations each time changes are indicated, mainly because the operability will continue to evolve up until observation execution. Likewise, not all impacts require replanning. 

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

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).

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).

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 or master background (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. 

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).