NIRSpec Multi-Object Spectroscopy

One of the showcase observing modes of JWST is the NIRSpec multi-object spectroscopy (MOS) mode using the micro-shutter assembly (MSA).  The MSA can obtain simultaneous spectra of many science targets within a 3.6' × 3.4' field of view.

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See also:  JWST MOS SpectroscopyNIRSpec MSA Planning Tool, MPT, and NIRSpec MOS Proposal Checklist (internal)

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 four 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 four 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 four 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 three-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 three-shutter slitlet configuration

A zoomed in view of several three-shutter slitlets configured open on MSA science targets of interest in the Hubble Ultra Deep Field (HST ACS F606W image).


Properties of the MSA

See also: NIRSpec Micro-Shutter AssemblyNIRSpec MOS Recommended Strategies

The MSA consists of four quadrants, each with 171 rows of 365 shutters, totaling ~250,000 shutters.  The four 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 0.069" wide, so that the shutter pitch (center-to-center distance) is 0.27" in the dispersion direction and 0.53" in the spatial (cross-dispersion) direction.

The area in each MSA quadrant is about 95" in the dispersion (X) direction by 87" in cross-dispersion (Y), and the total extent of the MSA sky field of view is 3.6' × 3.4'. The MSA quadrants are separated from one another by ~23" along the dispersion (X) axis (that is, between the inner edges of the tan-shaded shutter arrays shown in Figure 3 from the right edge of Q3 to the left edge of Q1, and similarly between Q4 and Q2). Note that this gap not the same as the gap between detectors in the dispersion (X) direction, which is ~ 18".  It is the detector gap that is responsible for missing wavelength information in spectral observations with the MSA. The MSA mounting plate also separates Q1 from Q2, and Q3 from Q4, by about 37" in the spatial (Y) direction. The fixed slits, located in this 37" gap between quadrants, are always open. 

Figure 3. NIRSpec MSA layout in the aperture plane

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. The detector array which is partially hidden in this figure by the MSA is really two detector arrays, with an 18" gap between them that is approximately aligned with the gap between quadrants Q1/Q3, and Q2/Q4.

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 seconds.

Some of the shutters in the NIRSpec MSAs are defective and either permanently "failed open" or "failed closed."  There are presently a small number (less than twenty) permanently failed open shutters; these are detrimental to science because spectra from spurious sources can overlap and contaminate science spectra.  Additionally, approximately 15% of the quadrant shutters are permanently failed closed or impacted by electrical shorts and therefore inoperable (see Figure 4).  

Of those shutters that are failed closed, there are three types:

  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;
  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 15% 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.

A few considerations for MOS observing:



Spectral configurations

See also: NIRSpec Dispersers and Filters

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

Disperser-filter combinationNominal resolving powerWavelength range
(μm)
G140M/F070LP1~1,000



0.701.27
G140M/F100LP0.971.84
G235M/F170LP1.663.07
G395M/F290LP2.875.10
G140H/F070LP~2,700

0.811.27
G140H/F100LP0.971.82
G235H/F170LP1.663.05
G395H/F290LP2.875.14
PRISM/CLEAR~1000.605.30

Wavelength range values presented here are approximate. Note that the nominal spectral ranges for medium and high-resolution dispersers may be shortened due to red-end detector cutoffs. The cutoff wavelengths depend on the target aperture location (slit or shutter). Detailed limits are found on the wavelength ranges and gaps pages for the IFU, FS, and BOTS, and in the ETC. Information on wavelength ranges for MOS, which depend on the position of the shutter in the MSA, can be determined using the MSAViz Tool.

MSA spectra are projected onto the two NIRSpec detectors (NRS1 and NRS2). In the NIRSpec MOS mode, some shutters will not capture the full spectral range at the high resolution, R ~ 2,700, configurations. This is because the right-most MSA shutters in MSA quadrants 1 and 2 (Figure 3) project the longest wavelengths beyond the right edge of detector NRS2 in the R = 2,700 configurations.  Alternatively, R~1000 spectra do not extend past the right hand side of NRS2 (see Figure 5). 



Detector wavelength gaps 

See also: NIRSpec MOS Wavelength Ranges and Gaps

In the MOS mode, there are gaps in spectral coverage caused by the physical distance between the two detectors, referred to as detector wavelength gaps. The range of wavelengths lost in the gap are different for different shutters in the MSA since the spectra from different shutters maps to different locations on the detectors. Unlike fixed slits (FS) and integral field unit (IFU) observations, which suffer wavelength gap losses only in the R ~ 2,700 high spectral resolution mode, all grating and filter combinations in the MOS mode have shutters that lose wavelengths to the 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 detector wavelength gap and acquire complete spectra of science targets. Also, it is possible to inspect MSA configurations designed for MOS observations using the MPT to ensure that wavelengths of interest fall into operable regions on the detectors (in areas unaffected by the detector gap or the long wavelength cutoffs, for instance).  The tool is called the JWST NIRSpec MSA Spectral Visualization Tool (MSAViz).

The detector wavelength gap discussed here is different from the gap between quadrants of the MSA shown in Figure 3.



Subarrays 

NIRSpec MOS mode exposures are only acquired in FULL frame 2048 × 2048 detector pixel readout; no subarrays are used.



Exposure specification

See also: NIRSpec Detectors,  NIRSpec Detector Recommended Strategies

NIRSpec MOS exposure times are tied to the timing of the detector readout patterns. There are four readout patterns available for NIRSpec MOS observations:

  • NRSRAPID 1
  • NRS 
  • NRSIRS2RAPID
  • NRSIRS2

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 (42.8 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, but unlike the traditional readout equivalent, NRSIRS2 has five 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)Users interested in determining which readout pattern is best for their science should refer to the NIRSpec Detector Recommended Strategies article.

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Options for dithering

See also: NIRSpec MOS Dither and Nod Patterns,  NIRSpec Dithering Recommended Strategies,  NIRSpec MOS Recommended Strategies

Most observations with JWST will require dithering. This is especially true for NIRSpec since the PSF is under-sampled at most wavelengths.  The NIRSpec MOS mode provides two options for creating offsets or dithers:



What do NIRSpec MOS data look like? 

Figure 5 shows NIRSpec MOS mode data acquired with a ground calibration test lamp using the R = 1,000 G140M/F100LP short wavelength spectral configuration. The four MSA quadrant spectra are shown. This observation was acquired with a special five-shutter calibration-only slitlet pattern that had three shutters open with two 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

 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 ~ 1,000) 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.


References

Böker, T. 2016 ESAC JWST "On Your Mark" Workshop (ppt) (pdf)
The NIRSpec Multi-Object Spectroscopy (MOS) mode  

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

Kutyrev, A.S., Collins, N., Chambers, J. et al. 2008 SPIE, 7010, 70103d
Microshutter arrays: High contrast programmable field masks for JWST NIRSpec

JWST Community Lecture Series - The NIRSpec MSA: Multi-object Spectroscopy with JWST; J. Muzerolle, Dec 13, 2016




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