Long Slit MOS Observations

NIRSpec long slit MOS observations can be specified directly in the MOS Observation APT template rather than creating them using the MSA Planning Tool. This method is primarily used for observations of extended fixed targets or moving targets when MSATA is not needed or not possible. Pre-defined long slit MSA configurations can be selected in the Configurations/Pointings table of the observation. The MSA Configuration Editor is not needed.

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Creating a Long Slit MOS Observation

Two pre-defined long slit MSA configurations are available in the NIRSpec MOS observation template. If no angle special requirements are placed on the observation, it will be executed with a feasible Aperture Position Angle (APA)* that is assigned to it by STScI.

Several TA methods are provided within the NIRSpec MOS observation APT template for use with one of the pre-defined long slits or 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 (Q4) field points. The quadrant 4 field point apertures can be used with their associated long slit MSA configurations directly in the observation template. The target specified in the Targets folder will be positioned at the reference point of the associated long slit aperture. 

The Quadrant 4 field points

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.) The MSA shutter operability for long slits positioned at these locations on the MSA is preferable from the standpoint of operable shutters. The two field points are sufficiently spaced to make it possible to obtain spectra that bridge the wavelengths lost due to the gap between detectors

Target Acquisition Considerations

See Also: TA Recommended Strategies

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.  

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 to 38" in APT.

Science Aperture and Long Slit MSA Configuration

If WATA, NONE or Verify_Only are used for target acquisition in the MOS observation APT template, the user can additionally select the Science Aperture (which includes options for the two defined field points). 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 long slit observation

Detailed 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 long slit MSA configuration for science with the Q4 Field Point. Normally, the matching MSA configuration for the science aperture should be selected.

Using the long slit configuration in a MOS Observation

In APT, migrate to the 'NIRSpec MultiObject Spectroscopy' template, and select any fixed or moving target defined in the proposal Targets Folder. 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' area of the observation template, it will then be possible to select the desired field point for the Science Aperture, as shown in Figure 1.  

Exposure entries added in the table on the Science Parameters area of the NIRSpec MultiObject Spectroscopy Template include a column for the MSA configuration. When clicked, the pull-down menu includes the two pre-defined long slit configurations. The pre-defined long slits can be used with the corresponding Q4 Field Point in the Science Aperture selection. All MSA configurations, including any that were custom made using the MSA Configuration Editor that reside in the same Observation should now be selectable. ALLOPEN and ALLCLOSED MSA configurations are also provided.

It is also possible to define long slits at locations other than the Q4 field points using the MSA Configuration Editor. Details on the use of the editor are presented in Custom MOS Observations. In brief, long-slits are created with a Cntrl-Left Click on a selected column in the MSA Configuration Editor. Any configuration created using the workflow described in this article can be edited by clicking on the Edit Config button in the Configurations/Pointings table of the Observation.

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 labeled 'Field 1' and 'Field 2'. Field 1 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.
*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.


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



Latest updates
  •  Created for new MPT in APT 2020.1.1
  •   Added note about the target positioning