NIRSpec Fixed Slits Spectroscopy

The JWST NIRSpec instrument fixed-slit spectroscopy mode (FS) utilizes 5 fixed slits for high-sensitivity, single object spectroscopy in the 0.6–5.3 μm wavelength range, using several grating-filter combinations and detector settings.

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See also: JWST Slit SpectroscopyNIRSpec Fixed Slit Spectroscopy APT Template

The JWST NIRSpec fixed slit (FS) spectroscopy options provide high sensitivity single object spectroscopy over the full 0.6–5.3 μm wavelength region where NIRSpec operates. NIRSpec fixed slits are designed for high contrast spectroscopy of both the faintest and brightest targets NIRSpec can observe. 

The FS apertures are illustrated in Figure 1, overplotted on an HST Advanced Camera for Surveys (ACS) wide field channel (WFC) F814 image of a young star.     

Figure 1. Sky view with JWST NIRSpec FS apertures

Four of the 5 NIRSpec FS apertures are shown overplotted on an HST ACS/WFC F814 image of the protostar IRAS 20324+4057. The target is within the S200A1 slit, shown at the upper right.

Properties of the FS mode

See also: NIRSpec Fixed Slits

The apertures for fixed FS, MOS, and IFU spectroscopy modes are all in the same focal plane in the NIRSpec instrument (called the slit plane). Figure 2 shows the location of all NIRSpec FS apertures in this focal plane, and a zoomed view of their positions and sizes.

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The 5 NIRSpec fixed slits are labeled S200A1, S200A2, S400A1, S1600A1, and S200B1. They are 0.2", 0.2", 0.4", 1.6", and 0.2" wide, respectively. Approximate dimensions of the NIRSpec fixed slits on the sky are presented in Table 1. The left-side slits in Figure 2 are referred to as the "A slits," and the right-side slit is the "B" slit. The S200B1 aperture is located on the opposite side of NIRSpec’s field of view from the "A" slits, and is there primarily for redundancy in the unlikely event that the NRS1 (left) NIRSpec detector fails. The S200B1 slit produces a truncated wavelength range when used with high resolution gratings (see Table 3) and as a result this aperture will not typically be used for science.

The S1600A1 aperture enables stable, high throughput spectroscopy in the FS spectroscopy mode. It is also optimized for observations of bright stars with transiting exoplanets in the NIRSpec bright object time-series (BOTS) spectroscopy mode.  


Table 1. The NIRSpec FS aperture sizes

FS aperture nameWidth (arcsec)Length (arcsec)
S200B1 0.23.2

The S200B1 slit is for redundancy and will not commonly be used for science.

Figure 2. NIRSpec FS apertures

NIRSpec FS apertures

Click on the figure for a larger view.

Locations of the JWST NIRSpec FS entrance apertures in the micro-shutter assembly (MSA) focal plane are shown. The red rectangle on the left is a zoomed section in the main figure that shows the "A"-side slits. A zoomed view of the S200B1 slit is shown on the right.

The FS spectroscopy mode can acquire the highest contrast, highest sensitivity spectra possible with NIRSpec. This is because the fixed slits are cut into the metal MSA support structure mounting plate, which is extremely opaque. Unlike the MOS or IFU spectroscopy modes, FS data will not be affected by the presence of contaminating objects through failed MSA shutters or by background signal due to the finite contrast of the MSA.  

In NIRSpec FS spectroscopy mode, subarray readouts can be used to decrease the detector readout time and observe brighter targets than is possible in the full detector readout used for the MOS and IFU spectroscopy modes. Consequently, the FS spectroscopy mode (and BOTS mode) can be used to observe the brightest targets possible with NIRSpec. Each of the 5 NIRSpec fixed slits have matched subarrays that encompass their individual spectra. The ALLSLITS subarray reads an area that includes all the fixed slits spectra. In this case, slits that do not contain an intended target can provide an independent measure of the background (see Figure 3).

What do NIRSpec FS data look like?

Figure 3 shows NIRSpec FS mode data acquired with a ground calibration test lamp using the R = 2,700 G140H+F100LP short wavelength spectral configuration. Spectra from the 5 fixed slits are visible in these ALLSLITS subarray data. The FS apertures are always open, so if a source or background emission falls through them, their signals will be acquired, even when the other NIRSpec observing modes are used.  

Figure 3. NIRSpec FS sample data

NIRSpec FS sample data

Click on the figure for a larger view.

An example of a calibration line lamp image of the NIRSpec FS mode data. Each NIRSpec FS has its own matched subarray; shown here is the ALLSLITS configuration which captures all 5 fixed slits in one exposure.

Spectral configurations

See also: NIRSpec Dispersers and Filters

All disperser and filter combinations available in NIRSpec can be used in the FS spectroscopy mode. Table 2 outlines the instrument configurations, resolutions and wavelength ranges that can be acquired with the NIRSpec "A slits."


 Table 2. NIRSpec A fixed slit instrument configurations, resolutions, and wavelength ranges

Disperser-filter combinationNominal resolving powerWavelength range



Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits on the wavelength ranges and gaps are found in the ETC.

The S200B1 slit is on the right side of the NIRSpec instrument slit plane and the wavelength range is truncated in the R = 2,700 grating settings. Table 3 shows the truncated spectral range for the S200B1 slit in the NIRSpec high spectral resolution mode. As a result of this decreased spectral range, the S200B1 slit is not recommended for regular science use.


Table 3. Truncated wavelength ranges in R = 2,700 settings for the S200B1 slit

Spectral configuration

Wavelength range

Detector wavelength gaps

See also: NIRSpec FS Wavelength Ranges and Gaps

Fixed slit spectra obtained with the high resolution R = 2,700 gratings span both NIRSpec detectors. Due to this and the physical separation between the detectors in the focal plane array, there will be small gaps in the spectral coverage for all the "A slits." A full description of the position of the gaps as well as wavelength ranges for each fixed slit aperture is available in the NIRSpec FS Wavelength Ranges and Gaps article.

Subarrays and readout modes

See also: NIRSpec Detectors, NIRSpec Detector SubarraysNIRSpec Detector Readout Modes and Patterns NIRSpec Detector Recommended Strategies

NIRSpec FS data can be acquired in FULL frame 2048 × 2048 detector pixel readout, or in the ALLSLITS subarray, or using subarrays that are matched to each slit. The matched subarrays have names corresponding to their respective FS apertures (Table 4). NIRSpec FS subarray exposures enable observations of brighter targets than are possible with the FULL frame detector readout because of their shorter frame read times. 


Table 4. The matched subarrays for the FS apertures.

FS aperture

Matched subarray name









S200A1 and S200A2



Four readout patterns are available for NIRSpec FS observations: NRSRAPID, NRS, NRSIRS2RAPID, and NRSIRS2. The first 2 are traditional readout mode patterns, and are similar to detector readout patterns for NIRCam and NIRISSNRSRAPID has a single frame per group (10.7 s FULL frame), and NRS averages 4 frames into a single group (42.8 s FULL frame). Those patterns including "IRS2" in their names correspond to improved reference sampling and subtraction mode (IRS2). These patterns intersperse reference pixels within the science pixel reads to improve noise characteristics during data processing. Additionally, NRSIRS2 averages 5 frames, instead of 4 as in NRSBoth factors result in longer exposures and higher data volumes for IRS2 patterns. Since they cannot be used with subarrays, IRS2 readout patterns are intended for long exposures of sources that are not too bright for FULL frame readout (see Table 5). The JWST Exposure Time Calculator (ETC) can be used to explore the range of FS exposure parameters and how they translate to exposure time and sensitivity. Strategies that can guide the user in the selection of detector readout parameters are discussed in the article NIRSpec Detector Recommended Strategies.

Expanded options for subarrays exist for the S1600A1 aperture (e.g., SUB512SUB1024ASUB1024B) so that very bright targets can be observed. These are described in more detail in the NIRSpec BOTS spectroscopy mode page.


Table 5. Detector frame readout time options for NIRSpec FS subarrays



Frame time (s)


2048 × 2048

10.7368 (traditional) or 14.5888 (IRS2)


2048 × 256






2048 × 64


SUB20482048 x 320.90156

Options for dithering

See also: NIRSpec FS Dither and Nod PatternsNIRSpec Dithering Recommended Strategies

Most observations with JWST will require dithering. Since the NIRSpec PSF is undersampled at most wavelengths, dithering is required to achieve nominal spectral and spatial resolution. The NIRSpec FS spectroscopy mode has several dither and nod options available. Dithers are offsets of the target position to even out or mitigate detector effects, cosmic rays, or improve spatial sampling. Nod offsets are also used in data processing to subtract nearby background flux.  

The options for the NIRSpec FS dithers include:

  • Primary nodding: 1 (single position with no slit dither), 2, 3, or 5 nods are available.
  • Subpixel offsets: These are used to improve spectral sampling of the line spread function or spatial sampling of the point spread function. Subpixel offset options are SPATIAL, SPECTRAL, or BOTH. Selecting these options will increase the number of acquired exposures.

The NIRSpec FS Dithers and Nods page provides an in-depth view of the available options briefly described above. Dithering strategies are discussed in NIRSpec Dithering Recommended Strategies.


Birkmann, S. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
Single Object Spectroscopy and Time Series Observations with NIRSpec

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Originally published