The JWST NIRSpec multi-object spectroscopy (MOS) mode provides multiplexing 0.6–5.3 μm spectroscopy capabilities over a 3.6' × 3.4' field of view. This mode uses tiny configurable shutters in the micro-shutter assembly (MSA) to acquire dozens to hundreds of spectra of astronomical targets within a single exposure. This is a very powerful feature for spectroscopic surveys. For example, potential use cases for the NIRSpec MOS mode include, but are not limited to: spectral characterization of the faintest objects in our universe, surveys to investigate galaxy formation and evolution, stellar population studies, star cluster formation, and the evolution and properties of extended solar system bodies.
The NIRSpec MSA consists of 4 quadrants of 365 × 171 shutters that can be individually opened and closed to create the spectral slit configurations for this multi-object spectroscopy mode. In Figure 1, the 4 NIRSpec MSA quadrants are plotted on a Hubble Space Telescope WFC3 F555W image of the Tarantula Nebula in the Large Magellanic Cloud.
Figure 1. The NIRSpec MSA quadrant fields viewed on the sky
A view of the 4 NIRSpec MSA quadrant fields over-plotted on a Hubble Space Telescope image of the Tarantula Nebula (HST WFC3 F555W image).
The NIRSpec MSA can be opened in contiguous columns of several shutters to create what are called “slitlets” to acquire MOS spectra of science sources of interest. Figure 2 shows a zoomed-in view of several 3-shutter slitlets, configured to open on (faint) sources, over-plotted on the Hubble Ultra Deep Field WFC3 UVIS image.
Figure 2. An example of an MSA 3-shutter slitlet configuration
A zoomed in view of several 3-shutter slitlets configured open on MSA science targets of interest in the Hubble Ultra Deep Field (HST ACS F606W image).
Properties of the MSA
The MSA consists of 4 quadrants, each with 171 rows of 365 shutters, totaling ~250,000 shutters. The 4 quadrants are labeled Q1, Q2, Q3, and Q4, as shown in Figure 3. Each MSA shutter is 0.20" × 0.46" (width in the dispersion direction × height).
MSA shutters are mounted on a fixed support grid. Support bars between all shutters are 0.06" wide, so that the shutter pitch (center-to-center distance) is 0.26" in the dispersion direction and 0.52" in the spatial (cross-dispersion) direction.
The area in each MSA quadrant is about 95" in dispersion by 87" in cross-dispersion, and the total extent of the MSA arrays is 3.6' × 3.4'. There are ~18" gaps between the 4 MSA quadrants in the dispersion axis (that is, between Q1 and Q3, Q2 and Q4 as shown in Figure 3). The MSA mounting plate separates Q1 from Q2, and Q3 from Q4, by about 36". Fixed slits, located in this 36" gap between quadrants, are always open.
Figure 3. NIRSpec MSA layout in the aperture plane
Four quadrants in the NIRSpec MSA aperture plane. Positions of the fixed slits and the IFU entrance aperture are shown on the left side of the diagram. The four MSA quadrants cover 3.6' × 3.4' on the sky.
To create an MSA configuration, the ~250,000 shutters can be individually selected to open or close. The NIRSpec MSA is configured to open sets of science shutters using a two-step process: (1) the MSA magnet arm sweeps across the MSA to open all shutters in the quadrants, then (2) the magnet moves back across the quadrants to close unused shutters and leave open those configured to observe specific science sources. This configuration process takes about 90 s.
Some of the shutters in the NIRSpec MSA s are defective and either permanently "failed open" or "failed closed." There are presently a small number (less than 2 dozen) permanently failed open shutters; these are detrimental to science because spectra from spurious sources can overlap and contaminate science spectra. Additionally, approximately 12% of the quadrant shutters are permanently failed closed and therefore inoperable (see Figure 4). There are 3 types of failed closed shutters: (1) some failed closed shutters are physically stuck closed, (2) some shutters used to be failed open but were deliberately blocked closed during the manufacturing process to limit detrimental contaminating spectra, and (3) some entire rows and columns of shutters in the MSA are masked closed due to electrical shorts in the MSA that prevent them from being addressed properly by the configuration magnet. The NIRSpec observation planning software will search for optimal MSA shutter science configurations and automatically plan around all failed shutters.
Figure 4. The MSA shutter operability map
The NIRSpec MSA shutter operability map shows failed closed MSA shutters as black lines or patches. Grey areas are MSA shutters that are operable for science configurations. Approximately 12% of NIRSpec MSA shutters are failed closed. These failed closed shutters are less detrimental to science than the failed open MSA shutters. The failed open shutters are not represented in this figure. NIRSpec observation planning software will automatically plan around the failed shutters. In this figure, the separation between the four quadrants is not properly represented.
The small size of individual NIRSpec MSA shutters means that detailed and careful planning must be carried out for most observations of compact sources. High quality astrometry (to 5–10 mas) is not required, but strongly recommended, particularly for sources smaller than the size of a shutter. Accuracy of target acquisition, and hence the flux calibration, is directly related to the input astrometric accuracy. The best astrometry is likely to come from HST images of the field or a JWST NIRCam mosaic image.
Larger or extended sources may be observed by configuring the MSA into a long slit or other suitable geometry. In this use case, detailed astrometry is not required, but pointing precision will be poorer.
All of the available disperser and filter combinations can be used in NIRSpec MOS mode. Table 1 below outlines usable instrument configurations, spectral resolutions and wavelength ranges.
Table 1. Spectral configurations available in NIRSpec MOS mode
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MSA spectra are projected onto the two NRS1 and NRS2 NIRSpec detectors. In the NIRSpec MOS mode, some shutters will not capture the full spectral range at the high resolution, R ~ 2700, configuration settings. This is because the right-most MSA shutters in MSA quadrants 1 and 2 (see Figure 3) project the long wavelength spectra off the right side of detector NRS2 in the R = 2700 modes. (See Figure 5 for a view of the MSA shutter data in R ~ 1000 mode).
Detector wavelength gaps
In the MOS mode, gaps in spectral coverage caused by the physical distance between the 2 detectors, referred to as detector wavelength gaps, depend on which MSA shutters are opened on sources. Unlike fixed slits (FS) and integral field unit (IFU) observations, which only have wavelength gap losses in the R ~ 2700 high spectral resolution mode, all grating and filter combinations in the MOS mode can have shutters that, if open, will lose spectra in the wavelength gap.
The NIRSpec MOS mode has a specialized MSA Planning Tool (MPT) within the JWST Astronomers Proposal Tool (APT) software. Using this planning tool, it is possible to create dither options to move targets by ~18" (or more) in the dispersion direction. This 18" is the approximate minimum dither distance necessary to span the wavelength gap and acquire complete spectra of science targets.
NIRSpec MOS mode exposures are only acquired in FULL frame 2048 × 2048 detector pixel readout; no subarrays are used.
NIRSpec MOS exposure times are tied to the timing of the detector . There are 4 readout patterns available for NIRSpec MOS 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 MOS observations of faint objects.
Additional information on NIRSpec MOS 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. The NIRSpec MOS mode 3 patterns for creating offsets or dithers.
Nodding: The telescope can be repositioned slightly between exposures to place the targets in different shutters within the slitlets of an MSA configuration. This is called nodding.
Fixed dither pattern: This option can be used to translate a planned configuration to a new location on the MSA, also resulting in a new spectral position on the detector.
Flexible dither pattern: For this option, the MPT finds an optimal pointing for multiplexing in an MSA configuration, then searches for subsequent optimal MSA configurations that maximizes observed targets at new offset positions.
The NIRSpec MOS dithering page provides an in depth review of the available options described only briefly here.
What do NIRSpec MOS data look like?
Figure 5 shows NIRSpec MOS mode data acquired with a ground calibration test lamp using the R = 1000 G140M/F100LP short wavelength spectral configuration. The 4 MSA quadrant spectra are shown. This observation was acquired with a special 5-shutter calibration-only slitlet pattern that had 3 shutters open with 2 closed in-between them. Two of the slitlet configurations are highlighted. Failed open shutters cause contaminating single shutter spectra. The MSA Planning software is designed to automatically optimize an MSA configuration around failed open shutters so that science spectra are not contaminated.
Figure 5. A view of NIRSpec MSA data
An example of NIRSpec MOS mode spectra taken with a calibration flat field lamp, providing uniform illumination, and an MSA shutter slitlet configuration using the G140M+F100LP spectral configuration. Two examples of the planned MSA spectral slitlets are highlighted at upper left. The fixed slits are always open and therefore always appear in MSA science mode exposures. Pointers also show zero-order images of the right spectral slitlets—these zero order slit images are seen in the medium resolution (R ~ 1000) spectral configurations and appear on detector NRS1. These images are associated with spectra on (approximately) the same row on the right side of the NIRSpec data on detector NRS2 (see pointers in yellow). The failed open MSA shutters appear as single shutter spectra (examples are highlighted at right). The gap between the NIRSpec detectors is not shown to scale.