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

See also: NIRSpec Fixed Slits Spectroscopy, JWST Slit Spectroscopy Roadmap, NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids

All 5 of 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. 

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Step-by-step APT instructions

We assume the observer has already defined Target(s) to be observed, loaded the NIRSpec FS template, and created an Observation Folder. Instructions for doing this are presented in the JWST Astronomers Proposal Tool Overview article.

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: This section 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 displayed by clicking on the NIRSpec Fixed Slit Spectroscopy tab. There are 3 additional tabs: Mosaic Properties, Special Requirements, and Comments, which are discussed at the end of this article.

IFU template tabs

Different sets of parameters are available when the following tabs (between sections 2 and 3) are selected: NIRSpec Fixed Slit Spectroscopy shows target acquisition and science observation parameters for the IFU. Mosaic Properties displays parameters required for mosaics. Special Requirements are used to place constraints on the observations, and Comments are for observing notes.

FS template section 1: Generic information

The following parameters are generic to all templates: Observation Number, Observation Label, Target name, and ETC Workbook Calculation ID (ETC Wkbk. Calc ID). The Target is defined in the Targets folder of the APT file. The ETC Workbook ID is entered in section 3 (Figure 1). 

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

See also:
NIRSpec Target Acquisition, NIRSpec Wide Aperture Target Acquisition, NIRSpec MSA Target Acquisition, 
NIRSpec Target Acquisition Recommended Strategies

For NIRSpec fixed slit observations, there are 3 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 Target Acquisition Parameters. For 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 100 milliarcsec or better are suggested to ensure that the acquisition target falls into the S1600A1 acquisition 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 2 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, parameters for section 3, as shown in Figure 1, are displayed. In this section, observers can define the remaining TA parameters for WATA, the Acq Subarray, AcqFilter, and the Acq Readout Pattern. The options for subarray are SUB32, SUB2048, and FULL. Options for filters are F110W, F140X, and CLEAR. The available Readout Patterns are NRSRAPID, and NRSRAPIDD6. These selections will be used to calculate the TA exposure time. 

The WATA procedure will always acquire a 3-group exposure using the AcqFilter 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 3-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. When WATA is not practical for a particular FS observation, then MSATA should be considered, as it incurs larger overhead and requires a more complex planning process with an aperture position angle (APA) assigned by STScI.

FS template section 4: science parameters

The Science Parameters are displayed in section 4 of the FS spectroscopy template, as shown in Figure 3. The Science Parameters include: the Slit, Subarray, 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. 

Slit and Subarray

See also: NIRSpec Fixed Slits, NIRSpec Detector Subarrays

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.  

Table 1. Available subarray options for each NIRSpec fixed slit

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

See also: NIRSpec FS Dither and Nod Patterns, NIRSpec Dithering Recommended Strategies, NIRSpec FS and IFU Mosaic APT Guide

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.

Create an exposure configuration 

See also: Understanding JWST Exposure Times, NIRSpec Detector Recommended Strategies

An exposure specification is created by using the Add button located beneath the Gratings/Filters dialog box in 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 options. After 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. 

 

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

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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. 

Comments 

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

 

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