NIRCam Coronagraphic Imaging
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.
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
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.
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.
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