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JWST's NIRSpec has five fixed slits (FSs) 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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Parent articleNIRSpec Observing Modes
See also: JWST Slit SpectroscopyNIRSpec Fixed Slit Spectroscopy APT Template

The JWST NIRSpec fixed slits (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, over-plotted on an HST Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) F814 image of a young star.     

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Figure 1. Sky view with JWST NIRSpec FS apertures

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Four of the five NIRSpec FS apertures are shown over-plotted 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 Slit spectroscopy, 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.

The five NIRSpec Fixed Slits are S200A1 1, S200A2, S400A1, S1600A1, and S200B1. These 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 slits in Figure 2 are referred to as the "A slits," and the right 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 FSs 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
margin10px 0 10px 0

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

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Figure 2. NIRSpec FS apertures

NIRSpec FS apertures

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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 Fixed Slit 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 five 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 five 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.  

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Figure 3. NIRSpec FS sample data

NIRSpec FS sample data

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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 five 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 Fixed Slits. 

 Table 2. NIRSpec A Fixed Slit instrument configurations, resolutions and wavelength ranges

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The S200B1 slit is on the right side of the NIRSpec instrument 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

DK: I changed the wavelength range values to be consistent with the values for the S200B1 slit shown in

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 type 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.


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 type A slits. The S200B1 slit is only affected by this in the G140H/F070LP grating/filter combination, but has incomplete spectral coverage to begin with (see Table 3).  When the two S200A1 and S200A2 slits are used in conjunction with each other, they mitigate the wavelength gap for full spectral wavelength coverage. Spectra at wavelengths that fall in the detector gaps are not recoverable in FS spectroscopy mode when using the S400A1 or S1600A1 slits.  Table 4 outlines the wavelength gap spectral ranges for the A slit R = 2700 instrument configurations in the FS mode. Spectra obtained with the medium resolution gratings or the PRISM do not have a wavelength gap because the complete spectrum will fall onto one detector (NRS1 for the A slits and NRS2 for S200B1; see Figure 5 in the NIRSpec Multi Object Spectroscopy page). 

Table 4. Wavelength gap ranges for the A slits 

FS apertureSpectral configurationDetector wavelength gap range
S200A1  G140H/F070LP   None
 G140H/F100LP 1.302–1.339
 G235H/F170LP 2.182–2.244
 G395H/F290LP 3.685–3.789
S200A2 G140H/F070LP   None
 G140H/F100LP 1.347–1.385
  G235H/F170LP 2.259–2.321
 G395H/F290LP 3.815–3.919
S400A1 G140H/F070LP   None






 G235H/F170LP 2.203–2.265

The G140+F070LP configuration in all slits of NIRSpec FS mode does not have a gap in wavelength because the science data lie all on one detector (the left detector, NRS1).

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, in the ALLSLITS subarray, or using subarrays that are matched to each slit. The matched subarrays have names corresponding to their 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 two 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 four 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 five frames, not four, as in NRS. Both 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 under-sampled 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.

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

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Updated July 10, 2017

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Published January 3, 2017



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