NIRCam Coronagraphic Imaging
JWST's NIRCam features Lyot coronagraphs that allow high-contrast imaging (HCI) at 2–5 µm wavelengths and sub-arcsecond inner working angles.
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The NIRCam coronagraphs feature 3 round and 2 tapered bar occulting masks (or occulters) in the focal plane and 2 apodizing masks (Lyot stops) in the pupil plane, which suppress the diffracted light of bright point sources and reveal much fainter nearby objects. (See the HCI Optics article for an overview of JWST's HCI techniques and capabilities.) One Lyot stop is used with the round occulting masks, and the other Lyot stop is used with the bar occulters. Two occulting masks (one round and one bar) are tailored for the short-wave (SW) channel, and three (2 round and 1 bar) are tailored for the long-wave (LW) channel.
The Lyot stops are metallic patterns deposited photolithographically onto optical wedges mounted in NIRCam's pupil wheels. When inserted into the optical train, the wedges shift the optical path so that the coronagraph optical mount (COM), which houses the occulting masks and normally lies outside the imaging field of view, is projected onto the short-wave (SW) and long-wave (LW) detector arrays (Figure 1).
Coronagraphic imaging is performed only with NIRCam's module A. (Redundant coronagraphs in module B are not presently used.) Dual-channel (i.e., SW and LW) images are recorded simultaneously, regardless of which occulting mask is used. The SW channel is considered to be the primary coronagraphic channel if the SW round (MASK210R) or SW bar (MASKSWB) occulter is selected. Likewise, the LW channel is considered to be the primary coronagraphic channel if a LW round (MASK335R or MASK430R) or LW bar (MASKLWB) occulter is selected.
Acquisition of bright coronagraphic targets is performed in the primary coronagraphic channel using one of several neutral density (ND) squares that share the focal plane with the occulting masks and provide ~7.5 magnitudes of attenuation (i.e., optical density ~3). Acquisition of fainter targets can be performed without using the ND squares. The filter used for target acquisition (TA) is fixed for each occulting mask, so the mode of TA depends on both the brightness of the target and the choice of occulter. Full-frame SW and LW astrometric confirmation images may be obtained before and after TA to allow confirmation of the small angle maneuvers that position the target behind the occulter, as well as precise registration of any sources imaged in subsequent coronagraphic subarray exposures.
Occulting masks
See also: NIRCam Coronagraphic Occulting Masks and Lyot Stops
Table 1 lists the shape, nominal wavelength range, and inner working angle (IWA) of each of NIRCam's 5 coronagraphic occulting masks.
The IWA is formally defined as the half-width at half-maximum (HWHM) opacity of a mask, i.e., the distance from the center of the mask to a point where the mask's opacity reaches 50% of its asymptotic value. It is commonly interpreted as the smallest angular distance from a bright object at which a faint object can be detected. The 3 round masks are designed such that IWA = 6λ/D, where λ is the nominal observed wavelength of each mask (2.1 μm, 3.35 μm, and 4.3 μm, respectively) and D is the 6.5 m diameter of JWST's aperture. The IWAs of the 2 tapered bars (measured perpendicular to the bar axis) vary end-to-end by a factor of 3, but the occulted star is located by default at IWA = 4λ/D, where λ is central wavelength of the bandpass filter used in the primary coronagraphic channel (Figure 2). Consequently, the nominal ranges of IWA for the bars are substantially narrower than their end-to-end HWHM ranges. (IWAs outside the nominal ranges can be accessed via specified pointing offsets along the bars, however.) The nominal IWAs of the bars are smaller than their round-mask equivalents, but this advantage is offset by the diminished fields of view at position angles approaching the long axes of the bars.
Table 1. Characteristics of NIRCam occulting masks
Mask | Shape | Wavelength range | IWA | |
|---|---|---|---|---|
(nominal)1 | (nominal)1 | (HWHM)2 | ||
| MASK210R | round | 1.82–2.12 μm | 0.40" | |
| MASK335R | round | 3.0–3.56 μm | 0.64″ | |
| MASK430R | round | 4.10–4.60 μm | 0.82″ | |
| MASKSWB | bar | 1.7–2.2 μm | 0.23″–0.27″ | 0.13″–0.40″ |
| MASKLWB | bar | 2.4–5.0 μm | 0.32″–0.61″ | 0.29″–0.88″ |
1 The nominal wavelength ranges and IWAs are defined, respectively, by the bandpasses and central wavelengths of the optical filters permitted for use with each occulter in the primary coronagraphic channel.
2 The HWHM ranges for the bar occulters reflect the values at the opposite ends of each bar. IWAs outside the nominal ranges are accessible via specified pointing offsets from the default target positions.
Although MASK210R and MASKSWB provide the smallest IWAs for SW imaging, the IWAs of the simultaneously recorded LW images will be larger because of the intrinsically larger point spread function (PSF) and possible detector saturation. At the longest wavelengths, the smaller masks will block the cores and innermost speckles associated with an unocculted PSF. The Exposure Time Calculator (ETC) can be used to assess the coronagraphic PSFs under such circumstances.
Fields of view
Simultaneous SW and LW coronagraphic images are recorded using either the full detector arrays or subarrays centered on the occulted target.
The fields of view (FOV) of the subarrays depend upon whether the SW or LW channel is the primary coronagraphic channel (Figure 3). The subarrays in the primary channel encompass 20″ × 20″ squares centered approximately on each occulting mask, except for MASKLWB, which features a 24″ × 15″ FOV to better accommodate the filter-dependent field points along the bar. (See Table 1 of NIRCam Coronagraphic Imaging APT Template.) Because (a) the pixel dimensions of the simultaneous SW and LW subarray images must be the same and (b) the pixel scale of the SW channel is approximately half that of the LW channel, the angular FOV of the subarray in the secondary channel will not be the same as that of the primary channel. For example, if the LW channel is primary, then the SW FOV is a quarter of the LW FOV. Conversely, if the SW channel is primary, the LW FOV will be 4 times larger than the SW FOV. Note that projected location of MASK430R is very close to the vertical gap between the SW detectors (Figure 1), so the SW FOV is effectively shifted by half its width to one side of that mask.
Click on the figure for a larger view.
Comparative fields of view of the coronagraphic subarrays when the SW channel (left) and the LW channel (right) is the primary coronagraphic channel. The pixel dimensions of each subarray are fixed for each associated pair of SW and LW exposures, so the angular sizes of the FOV vary with the corresponding pixel scales (0.31″/pixel for SW; 0.63″/pixel for LW). The black regions reflect the footprints of the ND squares and unilluminated areas beyond the physical boundaries of the COM. The green bars in the LW panels when the SW channel is prime indicate the boundary between illuminated regions astride the lower edge of the COM. Note that MASK430R appears at the right edge of the SW subarray due to the projected proximity of the mask to the vertical gap between the SW detectors (Figure 1). Figure made with WebbPSF.
Filters for coronagraphic imaging
Figure 4 shows JWST+NIRCam throughput curves for the subset of NIRCam filters that is available for coronagraphic imaging. The number and types of allowed primary- and secondary-channel filters depend upon the selected occulting mask, as described in NIRCam Filters for Coronagraphy.
Expected and on-sky performance
The NIRCam coronagraphs are designed to detect protostellar, protoplanetary, and debris disks around bright stars, as well as warm Jupiter-type exoplanets that are ~10 times less massive than those detected so far from the ground. The detectability of a faint companion depends primarily on the contrast (flux ratio) between the companion and its bright host as a function of their apparent separation (see HCI Contrast Considerations). Higher contrast sources are detectable at larger apparent separations. Detections are improved by observing strategies, such as obtaining multiple observations at different roll angles, and by data analysis techniques.
Figure 5 compares the pre-flight predicted contrast ratios for 5-σ detections of a faint companion near a bright host against actual contrast curves measured during JWST commissioning. The curves show that the curves generated by the Exposure Time Calculator (ETC) are adequate for proposal preparation in the vast majority of cases.
Click on the figure for a larger view.
Predicted and measured contrast ratios required for a 5-σ detection of a faint companion as a function of apparent separation from a bright host. Curves are shown for each round and bar occulter after subtraction of 2 images obtained at different roll angles (+5° and -5°) for speckle suppression. A position uncertainty of 10 mas and wavefront error of 10 nm between rolls were assumed. NIRCam should achieve almost 12 and 18 magnitudes of suppression at 1" and 4" from the central bright object, respectively.
- Top left: Pre-flight predictions of Beichman et al. 2010. The contrast curves are overly optimistic at large separations in the background and/or detector readout noise limited regime.
- Bottom left: Pre-flight predictions of Perrin et al. 2018 for MASK335R/F335M and 3,600 s exposure time. The predicted curves are close to those measured on-sky at separations < 1", but they flatten prematurely along the fundamental noise floor limit.
- Right: Measured on-sky contrast curves from Girard et al. 2022 for MASK335R/F335M and 3,300 s exposure times. The dashed green curves shows the discrepancy with Beichman et al. 2010 beyond ~1.5" where the curve flattens in the detector readout noise limited regime.
References
Beichman, C. et al. 2010, PASP, 122, 888
Imaging Young Giant Planets From Ground and Space
Carter, A., et al. 2023, ApJ Letters, 951, 1
The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems I: High Contrast Imaging of the Exoplanet HIP 65426 b from 2-16 μm
Girard, J. H., et al., 2022 (Commissioning, # 1441), SPIE, 121803Q
JWST/NIRCam Coronagraphy: commissioning and first on-sky results
Green, J. et al. 2005, Proc. SPIE 5905, 0L
High contrast imaging with the JWST NIRCAM coronagraph
Kammerer, J., et al., 2022 (Commissioning), SPIE, 121803N
Performance of near-infrared high-contrast imaging methods with JWST from commissioning
Krist, J. et al. 2010, Proc. SPIE 7731, 3J
The JWST/NIRCam coronagraph flight occulters
Krist, J. et al. 2009, Proc. SPIE 7440, 0W
The JWST/NIRCam coronagraph: mask design and fabrication
Krist, J. et al. 2007, Proc. SPIE 6693, 0H
Hunting Planets and Observing Disks with the JWST NIRCam Coronagraph
Perrin, M., et al., 2018, SPIE, 1069809
Updated Optical Modeling of JWST Coronagraph Performance, Stability, and Strategies