The JWST NIRSpec multi-object spectroscopy (MOS) mode uses nearly 250,000 configurable micro-shutters to form slitlets for simultaneously collecting spectra on multiple science sources.

The JWST NIRSpec MOS observing mode uses a micro-shutter assembly (MSA) that is comprised of nearly 250,000 configurable micro-shutters that can be opened in columns in the cross-dispersion direction to form spectral "slitlets." These MSA spectral slitlet configurations create the MOS slit masks, and allow for simultaneous collection of dozens to hundreds of source spectra. The MSAs were fabricated at NASA Goddard Space Flight Center.

Properties of the MSA

The NIRSpec MSA has 4 quadrants of configurable shutters. These are labeled quadrants 1 to 4, as seen in Figure 1. Each quadrant is comprised of 365 shutters in the dispersion direction (the vertical direction in Figure 1) and by 171 shutters in the cross-dispersion direction (horizontal direction in Figure 1). The total extent of the MSA quadrants is approximately 3.6' × 3.4'. There are gaps of approximately 23" (in the dispersion direction) and 37" (in the cross-dispersion direction) between the 4 MSA quadrants. Each quadrant has an active region of ~95" × 87" where shutters are available for source placement.  All dimensions provided above are averages, based on laboratory measurements on the ground.

Each shutter has an open area of approximately 0.20" × 0.46", with an ~0.07" separating wall on all sides. The total shutter pitch (center-to-center distance) is ~0.27" in the dispersion direction and ~0.53" in the spatial (cross-dispersion) direction. The shutter walls extend away from the shutter door, in the direction from which light enters, creating a “shield” to minimize shutter-to-shutter contamination and a “crate” for the shutter door to open into (see Figures 2 and 3).

In the MSA shutter configuration, all shutters are opened by sweeping the magnet arm across the MSA from bottom to top for the view shown in Figure 1. Then, the shutters are left open or closed during a return sweep, from top to bottom, of the moving magnet arm. Shutters that are configured open are latched in place by an electrostatic charge on the sidewalls of the shutter crate until they are reconfigured. Each shutter has a row electrode and a column electrode so that it may be addressed individually, allowing a tailored slit mask for each observation. The planning of MSA observations is done to minimize slitlet reconfigurations in order to maximize the operational lifetime of the MSA hardware.  

MSA unusable shutters

See also: NIRSpec MOS Recommended Strategies, NIRSpec MSA Shutter Operability

The MSA is deliberately oversized compared to the instrument field stop in the NIRSpec fore-optics, meaning that ~9.6% of the nearly 250,000 configurable micro-shutters are vignetted. For on-sky science programs, these shutters are unusable (although they are illuminated by internal calibration lamp exposures).

The complexity of the densely-packed, individually-addressable apertures inevitably leads to some shutters being inoperable. Overall, ~17.5% of unvignetted shutters  are not fully operable: some are permanently failed closed, others are purposely masked closed to prevent damage from electrical shorts within the MSA control electronics, and a very small number are permanently failed open. Additional details are provided below:

• Failed closed shutters
  These shutters simply do not open when commanded. Failed closed shutters do not degrade the obtained science spectra, but do cause an inconvenience during configuration planning. Failed closed shutters restrict the possible locations of science source slits and can limit the area that is available to use for target acquisition (TA) reference stars. When multiple failed closed shutters occur in the same region, or if the input catalog is sparse, this may decrease overall MOS multiplexing.
   
• Masked closed shutters
  Several dozen electrical shorts have appeared on the MSA over the years during ground testing and flight, caused by particulate contamination of the complex shutter control circuits. To protect the MSA from unsafe currents, a identified shorts must be removed from the circuit path by masking out an entire row or column in the affected quadrant using a special short mask applied directly at the hardware level. These masked shutters, like the failed closed shutters, are unavailable during configuration planning for science or TA.
   
• Failed open shutters
  These shutters either do not close when commanded or have no shutter door. Failed open shutters are always open to the sky and always acquire spectra. They pose a risk of contamination for NIRSpec MOS and IFU mode science because spectra from a source or background that falls in the aperture will always be collected. The failed open shutters are more detrimental to spectral MOS multiplexing and data quality than failed closed shutters, because failed open shutters prevent use of other open science shutters in a row of the MSA. When planning MOS slitlet configurations, the locations of science source slits can be limited to those that have no spectral overlap with failed open shutters. In the IFU observing mode, these failed open MSA shutters can be mitigated by dithering, by moving targets to new locations in the IFU field (and on the detector), or by acquiring IFU leakage correction exposures to remove the detrimental flux effects.

The characteristics of the MSA shutters have been calibrated throughout ground tests and flight. The location of failed open, failed closed, and short-masked shutters will be updated in the observation planning software as the population changes over time. 

Figure 5 also reveals several very low level ghosts (<0.01%) on detector NRS1 to the left of the MSA quadrants. When dispersed, this ghost emission is blue-ward of MOS mode spectra and is not expected to affect science performance.

MSA flux leakage

See also: NIRSpec MSA Leakage Subtraction Recommended Strategies

Ground testing of the NIRSpec MSA revealed that the shutters have a flux leakage problem (Figure 6) where light from bright field illumination can pass through closed MSA shutters at specific locations. This is the same issue that is discussed in NIRSpec MSA Leakage Correction for IFU Observations. Figure 6 shows the flat field images acquired with all MSA shutters configured closed to measure contrast (also shown in Figure 5), and overplotted is a zoomed region in MSA quadrant 4 that shows a very regular grid-like structure to the contrast illumination pattern. The precise instrument model knowledge of where the MSA shutters were located was compared with the image of illumination structure shown in Figure 6. As a result, it was determined exactly where the excess leakage flux comes from in the shutters.

Figure 7 shows a zoomed view of an MSA shutter, centered at (0, 0) in x and y coordinates. This is a super-sampled map of the leakage flux from the MSA; the color scale shows where the shutters are extremely opaque (blue) and where a moderate amount of flux leaks through to the spectrograph (red). The colors presented are relative flux levels, and the red color corresponds to a leakage level (measured closed flux ⁄ measured open flux) of ~0.004%. The average leakage structure is very regular and repeatable across most shutters in the MSA. The map in Figure 7 reveals that the majority of the MSA flux leakage arises from the location of the the bar separating shutters in the y direction (cross-dispersion direction). Additional investigation is being carried out to understand the origin of this MSA flux leakage problem.

Although the flux leaking through an individual MSA shutter is less than one part in 10,000, the leakage is a cumulative effect from the over 700 MSA shutters in the dispersion direction. As a result, the MSA flux leakage can accumulate to a level that is closer to one part in ~50 (2%), on average. Figure 8 presents a dispersed NIRSpec prism flat field image that shows science-like slitlet configuration, plus the elevated flux level from MSA flux leakage. The excess flux from the MSA leakage can affect background flux levels MOS mode observations, but the available NIRSpec MOS nod offset options can help to subtract excess flux from science observations. Additionally, the MSA flux leakage can affect the sensitivity of IFU observations, so the concept of IFU leakage correction exposures was adopted to mitigate the issues. The MSA Leakage Subtraction Recommended Strategies article discusses different strategies for different science cases.

References

NIRSpec Multi-Object Spectroscopy 
Presentation by T. Boeker at ESAC JWST "On Your Mark" Workshop, 26-28 Sep. 2016.

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