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, but are not limited to: spatially resolved kinematics and emission lines in distant galaxies, resolved spectroscopy of galactic nuclei or individual star clusters in nearby galaxies, detailed studies of selected sub-areas of galactic star forming regions, emission characteristics in resolved proto-stellar disks, 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). This is achieved by optically rearranging all spatial resolution elements within the field of view into a stacked configuration (shown in Figure 4).
In the case of the NIRSpec IFU, optical realignment of the spectra is accomplished by an “image-slicing” technique using mirrors. The resulting IFU 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 Integral Field Unit page.
Figure 1. NIRSpec IFU 3" × 3" field of view on the sky
The 3" × 3" NIRSpec IFU field of view shown over-plotted in blue on a Hubble Space Telescope WFC3/UVIS F625W image os Supernova 1987A within the Large Magellanic Cloud.
Properties of the IFU mode
Figure 2 shows the alignment of the NIRSpec IFU image slices on the detector as they would appear in imaging mode, and how the slits map to their spatial position within the 3" × 3" IFU field of view. Each of the 30 slices are 0.1" wide and 3" in length on the sky.
Figure 2. The 3" × 3" NIRSpec IFU image slices and sky view
|The JWST NIRSpec IFU image slices as positioned on the detector in imaging mode (left). In order to recreate the observed source image, the slit images are extracted and combined (right). The numbers in both of the panels show the location of the IFU slice image on the detector (left), and its corresponding spatial position within the observed field of view (right). The spectra for each slice can be combined to form a three-dimensional "data cube" that contains a source image for each spectral resolution element. |
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).
Figure 3. IFU entrance aperture location and characteristics
The location of the IFU entrance aperture in the MSA focal plane is highlighted in the very small red square, and a zoomed view is presented to the left. The IFU entrance aperture is a square cut in the metal mounting of the MSA. The square, ~3" × 3" in size, is the section of sky that is sampled by the image slicing mirror optics in IFU mode.
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.
- Failed open MSA shutters: The MSA has a population of shutters that are permanently or intermittently stuck in an open configuration (for an example of how this affects the data, see Figure 4). These failed open shutters always produce spectra of the sky, no matter the state of the MSA. For an IFU mode observation in a low density region of the sky, these shutters might only contain sky background flux from a blank field. However, in very crowded fields, the IFU spectra will likely be contaminated by the spectra of field objects through these failed open MSA shutters. If this characteristic is considered problematic for science data quality, the IFU aperture can be closed and a “IFU leakage” exposure or set of exposures can be acquired. Such observations can be used in post-processing to subtract the spectral contamination from failed open MSA shutters.
- MSA contrast: The MSA shutters have finite contrast and are not completely opaque. Flux contamination in IFU observations from finite contrast MSA shutters is referred to as "print-through." MSA shutter contrast is defined as the flux through an open MSA shutter divided by the flux through a closed MSA shutter; higher contrast numbers characterize more opaque shutters. MSA contrasts range from patchy regions of 2000–3000 in MSA quadrant Q3, to contrasts of 80,000+ (very opaque) seen in many regions across MSA quadrant 4. While the print-through is generally small for an individual shutter, it can be large across all 365 MSA shutters in a row where the flux is dispersed. This can result in a loss of sensitivity in IFU mode observations. The effect can be problematic for very faint IFU targets and IFU targets in crowded regions or with strong nebulosity. If this is likely to affect the sensitivity of science exposures, the IFU leakage exposures (as described above) can also be used to correct this contamination from the finite contrast MSA shutters.
Note: dithering can help mitigate the effects of individual failed open MSA shutters on IFU science exposures, but it will not improve the effect of the cumulative MSA print-through from the finite contrast shutters.
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
|Disperser/filter combination||Nominal resolving power||Wavelength range (μm)|
The NIRSpec IFU 0.1" spatial elements are projected onto the detector with a factor of 2 magnification in the dispersion direction. As a result, the undispersed IFU slice images are 2 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
There is a physical gap between the 2 NIRSpec detectors in the focal plane array. This affects NIRSpec IFU observations with the high resolution (R = 2700) 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. Lost wavelength ranges for the R = 2700 instrument configurations for the NIRSpec IFU mode are provided in Table 2.
Table 2. Detector wavelength gaps in NIRSpec IFU Mode
Detector wavelength gap range (μm)
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. All numbers are approximate and will be revised following post-launch commissioning tests.
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 4 readout patterns available for NIRSpec IFU observations:
The readout patterns are split over 2 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 4 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 4 frames averaged into a single group. 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).
Options for dithering
Most observations with JWST will require dithering to mitigate detector effects and improve sensitivity. 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 mitigate effects and improve sensitivity, but nods differ because the offsets are also used in data processing to remove astrophysical background flux.
Table 3. Summary of NIRSpec IFU dither and nod options
Two points separated by ~1.6" in both X and Y directions.
|Four-point dither ||Four points forming a box that's ~1.6" on a side. All points lie within one IFU aperture of each other, so there will be some overlap in their fields.|
Same as above, except these data will be processed differently by the calibration pipeline.
|Cycling dither pattern|
Up to 60-point cycling dither pattern (~1", 0.5" and 0.25" extents)
The NIRSpec Dithers and Nods page provides an in-depth view of the available options briefly described in Table 3.
What do NIRSpec IFU data look like?
Figure 4 shows NIRSpec IFU mode data acquired with a ground calibration test lamp using the R = 2700 G140H and F100LP short wavelength spectral configuration. Spectra of the 30 IFU slices are the horizontal bands in the figure. The FS apertures are always open, so data is acquired through them even in IFU mode. Also shown is the IFU image field (upper left), and in red a mapping of the slice positions to their dispersed spectral locations on the data view.
The spectra produced by the IFU are dispersed onto the same regions of the detectors used by the NIRSpec MOS mode, so the IFU and MOS modes are not used simultaneously. Pipeline algorithms transform the data read out from the detectors into 3D data cubes.
Figure 4. NIRSpec IFU example data
|An example of a NIRSpec IFU mode calibration line lamp image taken with the G140H and F0100LP spectral configuration. Thirty IFU data slices are shown as horizontal bands that span the two NIRSpec detectors in the upper and lower regions of the figure. Spectra in the middle are from the NIRSpec FSs. The FSs are always open and spectra are always acquired on the sky. The diagram in the upper left shows slices on the NIRSpec IFU field of view. The red arrows and numbers point to how the slices map from the field of view to the detectors. Examples of permanently or intermittently failed open MSA shutters, which result in spectra that overlap on IFU spectra, are highlighted by the yellow arrows. |