NIRSpec Integral Field Unit

The JWST NIRSpec integral field unit (IFU) hardware uses an image slicing mirror design to deliver spatially resolved imaging spectroscopy.  

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 NIRSpec's IFU can obtain spatially resolved imaging spectroscopy of a contiguous 3" × 3" area on the sky. To achieve this, the IFU uses specialized optics to reformat the spatial field of view and direct light to the spectrograph optics, which are shared with the multi-object spectroscopy (MOS) and fixed slits (FS) spectroscopy observing modes. This page describes NIRSpec hardware components that are specific to the IFU observing mode.

The IFU entrance aperture 

Light enters the NIRSpec IFU through a square entrance aperture that is machined into the metal separating the 4 quadrants of the micro-shutter assembly (MSA). Figure 1 shows the location of the IFU aperture in the NIRSpec field of view. The zoomed view to the left shows a photograph of the IFU entrance. The magnet arm that is used to configure the MSA has a small semi-circular extension—a "blocking lid"—which acts as a shutter and prevents light from entering the IFU when the IFU is not in use (shown in blue in Figure 2). This magnet arm moves to a different position to uncover the IFU entrance aperture when the IFU is selected for science.  

In any NIRSpec imaging observations used for target acquisition or position verification, the IFU will be configured open to also acquire field images through the IFU optics.

Figure 1. Hardware view of the IFU entrance aperture

Hardware view of the IFU entrance aperture

The NIRSpec aperture focal plane is presented to the right, and the location of the IFU entrance aperture is indicated. The photograph to the left is a zoomed view of the IFU entrance region. The aperture is shown as the small grey box. The semicircle to the left of the entrance aperture is a blocking lid extension on the MSA magnet arm. It moves to the left to uncover the IFU when it is in use, and to the right to prevent light leaks when it is not.
Figure 2. Positions of the MSA magnet arm

Positions of the MSA magnet arm

A diagram showing the NIRSpec focal plane and the 4 MSA quadrants (rotated by 90° relative to Figure 1). The IFU entrance aperture is shown as a small black square in the center bottom of the figure. The MSA magnet arm is shown in blue, at its location when the IFU is not in use, with the semi-circular blocking lid extension covering the IFU aperture. The magnet arm moves to the "IFU open" position in the lower part of the figure to uncover the entrance aperture (see Figure 1) when the IFU is used for science.

IFU optics

See also: NIRSpec Optics

In addition to the spectrographic components described in the NIRSpec Optics article, the IFU includes specialized optics to carry out 3-D imaging spectroscopy (see the optical schematics in Figures 3 and 4).  

After the light enters the IFU aperture, 2 re-imaging mirrors form a magnified image of the source field on the image slicer. The NIRSpec IFU image slicer (Figures 4 and 5) consists of 30 stacked mirror surfaces. Each is curved and tilted differently, sending the slice reflections on to 2 additional mirror arrays, the pupil mirrors, and slit mirrors (see Figure 4). The pupil mirrors are curved and tilted to align the slice images with the slit mirrors. The slit mirrors are also curved and tilted so that the aligned and stacked IFU slice images are directed towards the spectrograph entrance to the grating wheel assembly (GWA).

In this way, a 3.0" square region is rearranged into the stack of 30 spectra in the IFU data view. Each of the 30 spectra maps to a spatial region on the sky that is 0.1" wide and 3.0" high. Half of the spectra are directed towards the upper half of the detectors, away from the MSA frame and fixed slits, and the other half are directed towards the lower portion (see the IFU data view). The IFU optics magnify the image slices by a factor of about 2 in the dispersion direction, so that unresolved spectral emission features will appear 0.2" wide (2 pixels) on the detector. This gives about the same spectral resolution as the NIRSpec MOS mode and avoids undersampling the line spread function.

The IFU mapping from in-field slice position to detector spectral location is shown in the IFU data viewEach IFU slice puts the spectrum on a different part of the detector. Cross-talk between slices is less than 5%.

Figure 3. NIRSpec optical schematic with IFU location

Optical schematic for NIRSpec, with the position of the IFU optics shown. Here, MSA refers to the focal plane where the MSA shutters and IFU entrance aperture reside. © Dorner et al.
Figure 4. NIRSpec IFU optical schematic

NIRSpec IFU optical schematic

The optical configuration of mirrors in the NIRSpec IFU hardware. The "beam in" corresponds to the IFU entrance (from Figure 3), the slicer to the right is shown, and the beam out creates an image of the IFU slice.
Figure 5. Photograph of the IFU slicer mirror array

The IFU slicer mirror array hardware photograph

Photograph of the slicer mirror array. Note the change in slope near the center, which results in the slice images being directed away from the center of the detectors, above and below where the spectra from the Fixed Slits are located. A similar set of pupil mirrors and slit mirrors produces the output beam that is passed to the NIRSpec grating wheel assembly.


Closs, M., Ferruit, P., Lobb, D. R. et al. 2008, Proc. SPIE, v. 7010, 701011-1
The Integral Field Unit on the James Webb Space Telescope's Near-Infrared Spectrograph

Dorner, B., Giardino, G., Ferruit, P. et al. 2016, A&A, 592, A113 
A model-based approach to the spatial and spectra calibration of NIRSpec onboard JWST

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