NIRSpec Fixed Slit Spectroscopy APT Template

Detailed step-by-step instructions are available for filling out the JWST NIRSpec fixed slit spectroscopy APT template.

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All 5 NIRSpec fixed slit (FS) apertures can be used in the NIRSpec FS spectroscopy observing mode.  These apertures are always open, even if the Multi-Object Spectroscopy (MOS) mode or Integral Field Unit (IFU) mode is being used. In order to design a fixed slit observation, you have to complete a template in the  Astronomer's Proposal Tool (APT).  In this article, we describe how to fill out the NIRSpec fixed slits Template. 

Step-by-step APT instructions

We assume the observer has already defined Target(s)1 to be observed, loaded the NIRSpec FS template, and created an Observation Folder. Instructions for doing this are presented in the 2019-10-07_22-13-22_JWST Astronomers Proposal Tool Overview OLD.

The Fixed Slit template can be divided into 4 sections, as shown in Figure 1. 

  • Section 1: This section of the APT form shows that the NIRSpec Fixed Slit Spectroscopy template has been selected, along with a Target that was previously defined by the user. 
  • Section 2: The area contains key information that is calculated from the observations defined lower in the template. The observer cannot edit this section.
  • Section 3: In this section the observer enters target acquisition parameters.
  • Section 4: In this section the user specifies the science observation parameters.

Sections 3 and 4 are part of the 'NIRSpec Fixed Slit Spectroscopy' tab.  There are three additional tabs: 'Mosaic Properties', 'Special Requirements', and 'Comments', which are discussed at the end of this article.

1  Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.

Figure 1. The NIRSpec Fixed Slit Spectroscopy template

The NIRSpec Fixed Slit Spectroscopy template.  The parameters shown in sections 2 and 3 reflect the parameters found in the NIRSpec Fixed Slit Spectroscopy tab ( highlighted in blue).  The adjacent tabs lead to forms for the specification of a mosaicked observation, any special requirements for the observation, and observation comments.

FS Template section 1: Generic information

This section includes some information that is generic to all instrument templates.

The following parameters are also generic to all templates: Observation Number, Observation LabelObservations CommentsTarget NameETC Workbook Calculation ID, Mosaic Properties, and Special Requirements. The Target is defined in the Targets folder of the APT file. The ETC Workbook ID is entered in section 3 (Figure 1), while Mosaic Properties and Special Requirements, discussed briefly below, are made visible by clicking the tabs above section 3. 

FS Template section 2: Observation information

The fields presented in this section are generic to all templates. The values are calculated from the observations specified in sections 3 and 4. The article APT Observations contains more detailed information on the topics Visit Splitting, Duration, and Data volume. 

FS Template section 3: Target Acquisition Parameters

For NIRSpec Fixed Slit observations there are three options for target acquisition (TA): the Wide Aperture Target Acquisition (WATA), the procedure using MSA reference stars (called MSATA), and NONE. A complete description of the target acquisition procedures used to point the telescope with different JWST NIRSpec observing modes are addressed in the NIRSpec Target Acquisition article.

Section 3 in Figure 1 is where the user specifies the TA ParametersFor NIRSpec FS observations, the TA method called WATA (the default) is recommended. A detailed description of this TA method is found in the NIRSpec Wide Aperture Target Acquisition article. Absolute coordinate accuracies for the acquisition target on the order of 150 milli-arcsec or better are suggested to ensure that the acquisition target falls into the S1600A1 acquisition aperture. 

The option TA method = NONE is not recommended in most cases. In that case, the resulting pointing accuracy will be that delivered by the GS acquisition at the start of the Observation. For reference, the absolute pointing accuracy of JWST is expected to be 0.45 to 0.3 arcseconds (one sigma, per axis). Given the size of the 1.6" square aperture, a three sigma excursion in the blind pointing could place the source outside the aperture.

The WATA procedure typically starts with placing the science target in the S1600A1 aperture and centering it with onboard software. In this case, the Acq Target is the science target. It is also possible to use WATA with an offset target. In that case, the offset target is defined in the Target folder of the APT proposal, and can then be selected as the Acq Target in APT. When using an offset target, it is important to ensure that the relative astrometric accuracy of the two targets is sufficient to place the science target in the S1600A1 aperture. In either case, WATA is appropriate for targets that are point sources or nearly point sources.

When selecting WATA as the TA Method, section 3 appears as shown in Figure 1. In this section, observers can define the remaining TA parameters for WATA, the Acq SubarrayAcq Filter, and the Acq Readout Pattern. The options for Subarray are SUB32SUB2048, and FULL. The options for Filter are F110WF140X, and CLEAR. The available Readout Patterns are NRS, and NRSRAPIDThese selections will be used to calculate the TA exposure time. 

The WATA procedure will always acquire a three-group exposure using the Acq Filter and Acq Readout Pattern selected by the user. The JWST Exposure Time Calculator (ETC) in target acquisition imaging mode should be used to select appropriate exposure parameters. The desire is to produce a suitably strong signal (minimum of S/N = 20) in each group of the three group image without saturating the exposure. For quick reference, approximate magnitude ranges for S/N = 20 to saturation for WATA are shown in Table 2 of the article NIRSpec Wide Aperture Target Acquisition.

When selecting the MSATA option, the target acquisition parameters will be specified at the visit level of the Observation, not directly in the observation template. MSATA uses reference stars to accurately correct the pointing. MSATA should only be used when WATA is not possible, as it incurs larger overhead and requires a more complex planning process with an aperture position angle (APA) assigned by STScI.

Observers do not need to define reference stars in order to propose for NIRSpec FS science. That can be deferred until the final program update, as described in the MOS observing process article. Updated planning parameters for accepted programs will be specified by observers after the MSA-based Target Acquisition is assigned an orient (or APA) by the long range planning system. A detailed description of this TA mode is available in the NIRSpec Target Acquisition - MSATA article. 

FS Template section 4: Science Parameters

The Science Parameters are defined in section 4 of the FS Spectroscopy Template, as shown in Figure 3. The Science Parameters include: the Slit, Subarray, FS Dither Parameters, and the science exposure specification(s): the spectral configuration used for science, and the detector and exposure configuration options to specify the exposure time. 

Figure 3. Science Parameters section

Detail of the NIRSpec FS Spectroscopy Template showing the Science Parameters section.

Slit and Subarray

Figure 4 shows the options under 'Slit' and 'Subarray'. Once the slit is chosen, the possible subarrays are narrowed down to the options given in Table 1.  

Figure 4. Selecting a Slit and Subarray

Detail of the FS Template showing the available fixed slit options (top). Once a Slit is selected (in this case S400A1) the Subarray field defaults to the matching Subarray (SUBS400A1). Other options are shown in the menu (bottom). Table 1 lists all legal values for the Subarray for each Slit.

Table 1. Available subarray options for each NIRSpec fixed slit

SlitAvailable subarrays


S200A1 and S200A2


Note that the "S200A1 and S200A2" option is restricted for use with the high-resolution dispersers (G140H, G235H, and G395H) and their allowed FiltersThis option provides exposures using both slits to obtain complete wavelength coverage. The high resolution spectra map to both detectors. Using the S200A1 alone would otherwise result in wavelength gaps due to the space between detectors. With all other dispersers, the full wavelength coverage is achieved in a single exposure with the S200A1 slit. AExecution Order table will appear below the Gratings/Filters input table in the APT GUI. This table is not editable and shows the order of execution of exposure specifications of both slits.

APT will default to a matched Subarray once a Slit is selected.  The default is recommended unless the proposed science requires an alternative. Listed below are some considerations for choosing the subarray for use with fixed slit observations:

  • If S200A1 and S200A2 is selected, the Subarray options are ALLSLITS and FULL.
    • For very bright sources, using ALLSLITS may saturate the exposure in the first group. To work around this, use S200A1 with the SUBS200A1 subarray in one exposure and S200A2 and the SUBS200A2 in a separate exposure. Use the ETC to determine exposure settings and saturation potential. The NRSRAPID readout pattern is recommended in all sciences cases that have no constraints on data volume (which will be flagged in APT). This pattern helps to avoid saturation when using subarrays. 

  • For the S1600A1 aperture, several subarrays are available to allow for observing very bright targets. It is recommended to choose the largest subarray possible without saturating. Note that the smallest subarrays, SUB512 and SUB512S, only capture the full wavelength coverage for the Prism.

  • For each of the remaining fixed slits, there is a preferred (matched) Subarray of the same name, but it is also possible to select ALLSLITS or FULL frame detector readout. The ALLSLITS option reads the detectors for the whole region illuminated by all the fixed slits, and FULL reads the entire detector area. Selection of the FULL subarray option enables use of the NRSIRS2 or NRSIRS2RAPID readout options for Increased Reference Sampling and Subtraction. These patterns suppress 1/frequency noise and improve detector performance. NRSIRS2RAPID is the recommended Readout Pattern unless there are data volume issues in the program (these issues will be flagged by APT). 

Dither Parameters

Following the selection of the Slit and Subarray, the user must select Dither Parameters for the observation. The Primary Dither Positions offered are NONE, 2, 3, or 5 positions. The Sub-Pixel Pattern options are NONE, SPECTRAL, SPATIAL, or BOTH as shown in Figure 5. These patterns are illustrated and fully explained in the article NIRSpec FS Dither and Nod Patterns. Dithering in the spatial and spectral directions can improve signal-to-noise and sampling of the PSF. Recommendations for dithering FS observations are given in the article on Dithering Recommended Strategies.

Figure 5. Primary Dither Positions and Sub-Pixel Patterns

Detail of the FS Template where the available Primary Dither Positions and possible Sub-Pixel Patterns are shown.

Create an exposure configuration 

An exposure specification is created by using the 'ADD' button located beneath the Gratings/Filters area of section 4 of the FS Template, as shown in Figure 3. Each line in this field represents an exposure specification - a set of exposures acquired with the specified dither optionsAfter adding a new row in the exposure specification table, the science exposure parameters can be specified from pull-down menus. The parameters to define for each row in this field are listed in Table 2. Several of the science parameters in the table are described more fully in the NIRSpec Detector Readout article. The Exposure Time Calculator (ETC) should be used to determine the best values of these parameters to optimize the signal-to-noise of your observation. 

Recommendations about exposure parameter selection are given in NIRSpec Detector Recommended Strategies.

Table 2: Description of Science Parameters for a given exposure specification.

Exposure ParameterDescription
Grating/FilterThe grating/filter combination selected using the pull down menu. The article NIRSpec Dispersers and Filters describes all the available combinations for NIRSpec.
Readout PatternThe default value is NRS, but the available values are NRS, NRSRAPID, NRSIRS2, and NRSIRS2RAPID. These patterns are described in full detail in the article NIRSpec Detector Readout Patterns. Select the pattern that best suits your observation.
Groups/IntThe number of groups during an integration, where a group is the product of reading the detectors with a specific readout pattern. This number is determined from the ETC.  
Integrations/ExpThe number of integrations during an exposure, where integration is defined as the time between resets. This number is determined from the ETC.

This option is available to automatically add calibration exposures to a science exposure.  For the FS template, the only options are NONE or WAVECAL. NONE is the default and is recommended. Autocals can add significant overhead to an observation. The grating wheel sensor takes out any zero point wavelength shift (in post processing) due to slight variations in the position of the grating wheel. NIRSpec is required to deliver a wavelength accuracy to better than 1/8 of a spectral resolution element, or approximately 15 km/s for spectra taken with the high-resolution gratings. The instrument model wavelength calibration is expected to meet the wavelength calibration accuracy requirement, making Autocals unnecessary. Additionally, the grating wheel sensor removes any zero point wavelength shift (in post-processing) due to slight variations in the position of the grating wheel.

Other Tabs

Mosaic Properties

 NIRSpec's fixed slits may be used to obtain data for a region larger than their size by creating a mosaic pattern. The use of APT's mosaic tool is described in the article NIRSpec FS and IFU Mosaic APT Guide.

Special Requirements

A variety of observatory level special requirements may be chosen. 


The observation comments field should be used only to record observing notes.



Latest updates
    Figure 1 updated
    Figure 2 updated
    Minor updates to Table 2