The JWST integral field spectroscopy mode enables spectral imaging over small fields of view. The NIRSpec integral field unit (IFU) is designed for studies of astronomical targets that are extended over a few arcseconds, or for sampling small regions of more extended objects. Use cases for the NIRSpec IFU include, among others, spatially resolved kinematics and emission lines in distant galaxies, and atmospheric or topological features in extended moons or planets within the solar system.
The primary benefit of an IFU is the ability to obtain the spectrum of a contiguous, extended area on the sky (Figure 1). Optical realignment of the spectra is accomplished by an “image slicing” technique using mirrors in the IFU. The resulting slit images are dispersed without confusion from neighboring spatial elements. The 3" × 3" square field of view is dissected into 30 slices (Figure 2) and mapped onto optics to create 900 spatial elements within the field of view. NIRSpec spatial IFU elements are 0.1" × 0.1" on the sky, and can be dispersed using any of the NIRSpec gratings or the prism to acquire spectral imaging data cubes over the 0.6–5.3 μm wavelength range where NIRSpec is sensitive.
For further details on the IFU optics, please see the NIRSpec IFU instrument page.
Properties of the IFU mode
See also: NIRSpec Integral Field Unit
Figure 2 shows the 3" × 3" IFU field of view and how each slice maps to a different position on the detector. This is how the slices would appear in imaging mode on the detector. Each of the 30 slices are 0.1" wide and 3" in length on the sky, which corresponds to a height of 30 pixels on the detector. The spectral dispersion direction is also indicated.
The NIRSpec IFU entrance aperture is in the same focal plane as the micro-shutter assembly (MSA) shutters and the NIRSpec fixed slits (FSs). This is shown in a diagram of the NIRSpec entrance apertures in Figure 3. The IFU entrance aperture is a cutout in the metal mounting of the MSA. The aperture is opened during IFU operations, and closed when the IFU is not in use. The 3" × 3" field of view is sampled by the image slicing mirror in the IFU optics (black square in Figure 3).
In IFU mode, the NIRSpec MSA shutters are configured "all closed," and the IFU aperture is opened. Light from the NIRSpec IFU is dispersed onto the detector across the same pixel region as spectra obtained in the NIRSpec MSA mode. There are, however, two MSA instrument characteristics that affect IFU data and sensitivity:
See also: NIRSpec Dispersers and Filters
All grating-filter combinations and the prism can be used in the NIRSpec IFU mode. The instrument configurations, spectral resolutions, and wavelength ranges that can be used are outlined in Table 1. For the F070LP filter + G140 grating settings, the minimum measured wavelength is shifted and the blue-ward spectra lie off the detector. This is because the IFU aperture is toward the side of the NIRSpec focal plane (Figure 3).
Table 1. Instrument configurations, spectral resolutions, and wavelength ranges for NIRSpec IFU
The NIRSpec IFU 0.1" spatial elements are projected onto the detector with a factor of two magnification in the dispersion direction. As a result, the undispersed IFU slice images are two pixels wide, even though they project to an area of 0.1" x 3.0" on the sky. As a result of the magnification, the IFU has the same spectral resolving power as the MSA shutters and the 0.2" FSs.
Detector wavelength gaps
See also: NIRSpec IFU Wavelength Ranges and Gaps
There is a physical gap between the two NIRSpec detectors in the focal plane array. This affects NIRSpec IFU observations with the high resolution (R = 2,700) gratings because the spectra are long enough to span both NIRSpec detectors. Unfortunately, the wavelengths of the spectra which fall in the detector gaps are not recoverable in IFU mode.
The NIRSpec IFU Wavelength Ranges and Gaps article discusses in detail these gaps and provides figures that show the the gaps for dispersers G140H, G235H, and G395H.
The G140H/F070LP configuration in the NIRSpec IFU mode does not have a gap in wavelength because its spectra only fall on one detector (NRS1). Each IFU slice has a very slightly different wavelength gap range.
NIRSpec IFU exposures are only acquired in FULL frame 2048 × 2048 detector pixel readout; no subarrays are used.
NIRSpec IFU exposure times are tied to the timing of the detector readout patterns. There are four readout patterns available for NIRSpec IFU observations:
The readout patterns are split over two readout modes: (1) traditional and (2) 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 (42.9 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 (14.6s), but unlike the traditional readout equivalent, NRSIRS2 has five frames averaged into a single group (72.9 s). These IRS2 readout patterns improve performance and sensitivity in long exposure IFU observations of faint objects.
Additional information on NIRSpec IFU exposure specification and how this translates to exposure time and sensitivity can be found using the JWST Exposure Time Calculator (ETC). The NIRSpec Detector Recommended Strategies article provides guidance about the selection of appropriate readout patterns to use in different science cases.
Options for target acquisition
Target acquisition to place the science target into the small (3" x 3") IFU aperture is likely necessary for most science use cases. The default option, WATA, will suffice for most cases, but MSATA is offered when more precise positioning within the aperture is warranted. The MSATA process requires a selection of reference stars which in turn may require pre-imaging, or the availability of HST imaging to obtain accurate coordinates of nearby stars in the right brightness range. Considerations for the different methodologies for IFU target acquisition are discussed in NIRSpec Target Acquisition Recommended Strategies.
Options for dithering
Most observations with JWST will require dithering to mitigate detector effects and improve sensitivity. Since the NIRSpec PSF is under-sampled at most wavelengths, dithering is required to achieve nominal spectral and spatial resolution. For observations with the NIRSpec IFU, there are several options available, including dithers and nods. Both dither and nod options move science sources on the detector to help mitigate detector effects and improve sensitivity, but nods differ because the offsets are also pairwise subtracted in data processing to remove astrophysical background flux. IFU dithers are optimized to provide a factor of two sub-sampling of the PSF in both directions. Dithering is one strategy that can mitigate the effects of bright spoilers in the MSA on IFU data. These are others described in NIRSpec Bright Spoilers and the IFU Recommended Strategies.
Table 2. Summary of NIRSpec IFU dither and nod options
The NIRSpec Dithers and Nods page provides an in-depth view of the available options briefly described in Table 2. The NIRSpec Dithering Recommended Strategies article provides dithering advice tailored to different science goals. When choosing dithers, one should take into account target acquisition uncertainties, as described in the NIRSpec Target Acquisition Recommended Strategies article.
Options for background subtraction
Nodded offset exposures can be used for background subtraction. All available options are covered in NIRSpec IFU Dither and Nod Patterns. The article NIRSpec Background Recommended Strategies can help the user decide which options to use for their science case. The JWST ETC can be used to model and estimate the background in IFU mode, to determine whether background subtraction is needed and what observing parameters will yield the desired S/N on the science source. Bright science sources may not need this correction since the background is expected to be low at NIRSpec operational wavelengths. The JWST Background Model describes the different components of the background.
Figure 4 shows NIRSpec IFU mode data acquired with a ground calibration test lamp using the high resolution (R = 2,700) disperser G140H and filter F100LP. Spectra of the 30 IFU slices are the horizontal bands in the figure with labels 1 to 30 (on the left). The fixed slit apertures are always open, so data is acquired through them even in IFU mode. They are shown and labeled "Fixed Slits" in the figure.
Spectra produced by the IFU are dispersed onto the same region of the detectors used by the NIRSpec MOS mode, so the IFU and MOS modes are not used simultaneously. The red arrows point to the spectra of failed open MSA shutters that contaminate the IFU spectra.
Pipeline algorithms transform the data read out from the detectors into 3-D data cubes. One wavelength slice of a processed cube of simulated data is shown in Figure 4 in the lower left corner. Example wavelength and flux calibrated spectra of individual pixels are presented in the lower right corner of the figure.