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.
Parent article: NIRSpec Observing Modes
See also: JWST Slit Spectroscopy, NIRSpec Fixed Slit Spectroscopy Template APT Guide
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 FSs are designed for high contrast spectroscopy of both the faintest and brightest targets NIRSpec can observe. Example use cases for FSs spectroscopy include, but are not limited to: high sensitivity observations of very faint single targets such as high redshift quasars and isolated low mass brown dwarfs; bright sources such as solar system objects and young stars that would saturate in the NIRSpec multi-object spectroscopy (MOS); integral field unit (IFU) spectroscopy modes.
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.
Properties of the FS mode
See also: NIRSpec Fixed Slits
The apertures for FSs 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 FSs are S200A1 1, S200A2, S400A1, S1600A1, and S200B1 slits. These are 0.2", 0.2", 0.4", 1.6", and 0.2" wide, respectively. Approximate dimensions of the NIRSpec FSs 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 name||Width (arcsec)||Height (arcsec)|
The FSs spectroscopy mode can acquire the highest contrast, highest sensitivity spectra possible with NIRSpec. This is because the FSs 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 FSs 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. The FSs spectroscopy mode (and BOTS mode) can be used to observe the brightest targets possible with NIRSpec. Each of the five NIRSpec FSs have matched subarrays that encompass their individual spectra. The ALLSLITS subarray reads an area that includes all the fixed slits spectra.
See also: NIRSpec Dispersers and Filters
All disperser and filter combinations available in NIRSpec can be used in the FSs spectroscopy mode. Table 2 outlines the instrument configurations, resolutions and wavelength ranges that can be acquired with the NIRSpec A FSs.
Table 2. NIRSpec A FSs instrument configurations, resolutions and wavelength ranges
Disperser-filter combination Nominal resolving power Wavelength range †
G140M/F070LP 1 ~1,000 0.70–1.27 G140M/F100LP 0.97–1.84 G235M/F170LP 1.66–3.07 G395M/F290LP 2.87–5.10 G140H/F070LP ~2,700 0.81–1.27 G140H/F100LP 0.97–1.82 G235H/F170LP 1.66–3.05 G395H/F290LP 2.87–5.14 PRISM/CLEAR ~100 0.60–5.30
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
|Wavelength range |
Detector wavelength gaps
See also: NIRSpec FS Wavelength Ranges and Gaps
Fixed slits 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.
See also: NIRSpec Detector Subarrays
NIRSpec FS data can be acquired in FULL frame 2048 × 2048 detector pixel readout, the ALLSLITS subarray 2048 × 256, or subarrays that are matched to each slit (2048 × 64). The subarrays have names connected to their FS apertures, as shown in Table 5. NIRSpec FS subarray exposures allow for observations of brighter targets than possible with the FULL frame detector readout because of shorter frame read times.
Table 5. The matched subarray names for the FS apertures
Matched subarray name
S200A1 and S200A2
Table 6. Detector frame readout time options for NIRSpec FS subarrays
Frame time (s)
2048 × 2048
10.7368 or 14.5888 (IRS2)
2048 × 256
2048 × 64
|SUB2048||2048 x 32||0.90156|
FULL frame detector readout can use standard readout and the IRS2 option (as explained in the exposure specification section). Expanded options for subarrays exist for the S1600A1 aperture so that very bright targets can be observed. These are described in more detail in the NIRSpec bright object time-series, BOTS, spectroscopy mode page.
NIRSpec FS exposure times are tied to the timing of the detector readout patterns and subarrays. There are four readout patterns available for NIRSpec FS observations:
Readout patterns are split over two readout modes, traditional and improved reference sampling and subtraction (IRS2). The traditional mode, which is used for the NRSRAPID and NRS readout patterns, is similar to the detector readout for NIRCam and NIRISS. In FULL detector readout, NRSRAPID has a single frame (10.7 s), and NRS has four frames averaged into a single group (42.8 s).
The IRS2 mode, which is used for the NRSIRS2RAPID and NRSIRS2 readout patterns, intersperses reference pixels within the science pixel reads to improve noise characteristics achievable during data processing, resulting in longer frame times and higher data volumes. Like the traditional readout, the NRSIRS2RAPID is a single frame, and NRSIRS2 has five frames averaged into a single group. These IRS2 readout patterns improve performance and sensitivity in long exposure FS observations of faint objects. However, IRS2 readout patterns can only be used if the FULL frame is read, so FS subarray readouts cannot take advantage of the improved performance offered by IRS2.
Additional information on NIRSpec FS exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC) page. Strategies that can inform the selection of the detector readout parameters are discussed in the article NIRSpec Detector Recommended Strategies.
Options for dithering
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.
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 FSs 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.
Near Infrared Spectrograph (NIRSpec)
NIRSpec Observing Guidance
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