MOS Custom Configuration Process

It is possible to carry out MOS science observations of extended fixed targets or moving targets where MSATA is not needed or not possible. This is done using a pre-defined long slit MSA configuration, or a custom MSA configuration designed with the Manual Planner which can applied to a target pointing in the MOS observation template in APT without running the MSA Planning Tool (MPT)

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MOS observations of extended targets or moving targets are not created using the MSA Planning Tool (MPT) as for most MOS observations. Instead, the MOS observation template is filled out directly (as for other JWST observation modes). Two pre-defined long slit configurations are available in the observation template. Alternatively, custom MSA configurations designed with the Manual Planner can be applied to the observation in the template.

Several TA methods are provided within the MOS spectroscopy observing template for use with a custom designed MSA Configuration. For example, the JWST NIRSpec Wide Aperture Target Acquisition (WATA) observing sequence uses the S1600A1 “wide” aperture to perform target acquisition of moving science targets, stationary science targets, and stationary offset targets fainter than AB ~ 11.9 mag. WATA enables subsequent science observations that utilize any of the NIRSpec defined apertures (e.g., the IFU and any of the Fixed Slits).

There are two locations in quadrant 4 of the Micro-shutter Assembly that were defined to facilitate MOS observations of extended and moving targets. They are called MSA quadrant 4 field points. The quadrant 4 field point apertures can be used with their associated long slit MS configurations directly in the observation template.

Alternatively, to create observations in the MOS observation template using a custom MSA configuration, one must first create the MSA configuration. Even though a formal 'Plan' is not being created in this case, the Manual Planner must be used to define or import the desired MSA configuration. This is done in section 3 and 4 of the Manual Planner View. Use of the configurations created there will not be restricted to either the pointing or the exposure setup specified in those panes, but a pointing and exposure setup must still be specified to allow the Astronomers Proposal Tool (APT) file to be saved. It is not necessary to supply a Catalog or Candidate List, or to specify an Aperture Position Angle since a formal MSA Plan will not be created.

Bold italics style indicates words that are also parameters or buttons in software tools like the APT and ETC. Similarly, a bold style represents menu items and panels.

Quadrant 4 field points

Field Point Use with Target Acquisition

Two field points, Q4 Field Point 1 and Q4 Field Point 2, have been defined for MOS science observations of extended and moving targets. Currently the Q4 Field Point 1 location corresponds to the shutter labeled q4d250s25 on MSA4 (MSA quadrant 4), while Q4 Field Point 2 is located at q4d168s73, also in MSA quadrant 4.  (The numbers following the 'd' indicate the dispersion direction shutter position, while the numbers following the 's' indicate the spatial shutter location.) These positions on the MSA were designed such that the MSA shutter operability for long slits positioned there is preferable from the standpoint of operable shutters. Using WATA, the offset from the TA position in the wide aperture to the science position at Q4 Field Point 1 will typically be within the visit breaking distance for most orientations. Q4 Field Point 2, on the contrary, is much further away and was intended primarily to be used for more extended sources and moving targets with the TA methods Verify-Only or None. The two field points are sufficiently spaced to make it possible to obtain spectra that bridge the wavelength gaps caused by the gap between detectors. 

It is possible that these locations may be revised slightly in response to on-orbit changes in the MSA shutter operability; if that happens, users will be informed so they can revise their observations appropriately.

If WATA, NONE or Verify_Only are used for target acquisition in the MOS APT template, the user can additionally select the Science Aperture (which includes options for the two defined field points) (see Figure 1). This is enabled at the observation level in APT. In the example in Figure 1, the Q4 Field Point 1 science aperture has been selected. Additionally, the user would choose the corresponding Q4 Field Point 1 long slit for the MSA Configuration in the exposure specification.

Figure 1. MOS Observation Template selection of Quadrant 4 Field Points

This is a view of the MOS Observation template in APT. The Science Aperture selection of one of the Q4 Field Points is shown, along with the pull-down options for the MSA configuration for science with the Q4 Field Point. Normally, the matching MSA configuration for the science aperture should be selected.

Creating a long slit or custom configuration in the Manual Planner

To demonstrate the use of the Manual Planner to create a custom MSA configuration, the example of a long slit will be used, but any custom configuration may be designed simply by clicking shutters in the manual configuration pop-up editor. To create a long slit using the Manual Planner:

  1. Navigate to the Manual Planner by selecting the Observation Folder in the Tree editor
  2. Select the MSA Planning Tool tab
  3. Select the Planner tab
  4. Make sure the Manual Planning checkbox is checked
  5. Define an exposure setup (grating, readout pattern, groups, integrations, etc.)
  6. Click "New" below the "Configurations and Pointings" table to add a new entry.  
  7. Click "Edit new configuration" to open the manual configuration pop-up editor.
  8. Position the mouse at a shutter and control-left-click to open a long slit on both quadrants 4 and 3 of the MSA. Alternatively, open shutters by clicking on them. Shutters may be closed also by clicking on them.
  9. Be sure to give the configuration a unique name in the 'Pointings' tab of the Manual Planner pop-up designer window that allows its intended purpose to be easily identified.
  10. Specify the Base Pointing referenced to the center of the MSA. (This is necessary even when this pointing may not be used in the observation.)
  11. Click "OK" to return to the Manual Planner.
  12. Do not 'Finish Plan' in the Manual Planner; simply return to the observation template in the Tree editor and the custom-designed configuration will become available for selection.

It is also possible to save custom configurations as CSV files or to import a pre-existing CSV configuration file in the pop-up editor. Two long-slit MSA configuration CSV files for each of these two field points are provided as an example. They are LS-FIELDPT1.csv and LS-FIELDPT2.csv. However, there should never be a reason to use these CSV files as the canned long slits for these configurations are readily available now in the MOS observation template, as described above. When defining a custom configuration at a specific pointing, it is important to ensure that the target of the observation should be the same as the one used during manual planning. Note that the Manual Planner is referenced to the position at the center of the MSA by default.

Using the long slit or custom configuration in an Observation

Once one or more MSA configurations have been created in an Observation Folder in APT, a new MOS observation should be created in that folder setting the Instrument to 'NIRSpec', the Template to 'NIRSpec MultiObject Spectroscopy', and selecting any fixed or moving target defined in the proposal Target Folder. The target coordinates and instrument Aperture Position Angle (APA) do not need to correspond with the pointing or 'true angle to target' that were specified in the Manual Planner, and in the case of a moving target, the coordinates will of course not be known at the time the observation is defined. The 'TA method' should be set to WATA, NONE, or VERIFY_ONLY. In the science parameters pane, it will then be possible to select the desired field point for the Science Aperture, as shown in Figure 1. Q4 Field Point 1 is located about 28" from the S1600A1 "wide" aperture in which WATA is performed, so it should be straightforward to acquire a compact target using WATA and then move it the Q4 Field Point 1 for science observations. Q4 Field Point 2 is slightly over 55" from the S1600A1, and so WATA will not be possible unless the APA is restricted and an offset target is selected to reduce the required slew below the visit splitting distance. This field point, Q4 Field Point 2, is primarily intended for observations of extended moving targets, (e.g., Jupiter or a large comet), where a target acquisition would not be practical in any case. To support the use of the field points with WATA in more scenarios, the minimum visit splitting distance for cycle 1 for WATA has been enlarged from 30" to 38" in APT. 

Exposure entries added in the table on the 'Science Parameters' pane of the NIRSpec MultiObject Spectroscopy Template include an option to select the desired MSA configuration, which (when clicked) includes either of the two canned long slit configurations. The canned long slits can be used with the corresponding Q4 Field Point in the Science Aperture selection. Any custom configurations created that reside in the same Observation Folder should now be selectable, in addition to the ALLOPEN and ALLCLOSED MSA configurations. 

Finally, remember to complete the MOS Observation in the MOS APT template to indicate whether Confirmation Images are also desired.

Figure 2. Quadrant 4 field-point locations

A sketch of the locations of the two MSA Quadrant 4 field points labelled 'Field 1' and 'Field 2'. Field1 corresponds to "Q4 Field Point 1", and Field 2 corresponds to "Q4 Field Point 2". The dispersion direction is from NRS1 toward NRS2, as indicated by the arrow. A representative slit-like MSA configuration on the Field 2 position is shown. The detector gap is the area between the two green rectangular outlines of the detectors.


Proffitt, C. R. et al. 2017, JWST-STScI-005921
The Pre-Flight SI Aperture File, Part 4: NIRSpec



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