JWST/MIRI Coronagraphic Imaging

Updated 11 August 2025

JWST/MIRI coronagraphic imaging is a high-contrast mid-infrared technique that uses specialized masks and precise PSF calibration to directly detect faint astrophysical targets.
It employs four distinct coronagraph designs—including three 4QPMs and one Lyot spot—to achieve inner working angles as small as ~1 λ/D for probing exoplanets and circumstellar structures.
Advanced data processing methods such as microscanning, deconvolution, and reference differential imaging refine the PSF, enabling robust characterization of exoplanet atmospheres and disk substructures.

JWST/MIRI Coronagraphic Imaging comprises a set of high-contrast, mid-infrared imaging techniques using the James Webb Space Telescope’s Mid-InfraRed Instrument (MIRI). This system is designed for the direct detection and characterization of faint astrophysical targets—including exoplanets, circumstellar disks, and evolved stellar environments—at wavelengths (10–28 µm) where thermal emission dominates and atmospheric transmission prevents comparable sensitivity from the ground. MIRI achieves this by integrating advanced coronagraph concepts, precise optical alignment, and a rigorously characterized point spread function (PSF), complemented by optimized instrument calibration and state-of-the-art data processing.

1. Instrument Architecture and Coronagraph Designs

MIRI features four coronagraphs implemented within its Mid-InfraRed IMager (MIRIM) subassembly: three four-quadrant phase mask (4QPM) units optimized for central wavelengths of 10.65, 11.30, and 15.50 µm, and a classical Lyot spot coronagraph at 23 µm (Boccaletti et al., 2015). The optical relay partitions the entrance focal plane to allocate separate regions for conventional imaging, coronagraphy, and low-resolution spectroscopy (Wright et al., 2015, Bouchet et al., 2015).

The coronagraph masks (held in the filter wheel assembly, FWA) are aligned with precision pupil stops, relying on optical-mechanical tolerances such as <2% pupil shear and <1 mm focus misalignment to ensure that the stop is correctly positioned relative to the telescope’s exit pupil. Pupil alignment repeatability is specified to ~1 arcsecond (Wright et al., 2015).

The 4QPM design imparts a π phase shift between adjacent quadrants in the focal plane, causing destructive interference for the on-axis (stellar) light. This achieves rejection close to λ/D, providing inner working angles (IWA) as small as ~1 λ/D, whereas the Lyot spot mask is sized for an IWA of ~3.3 λ/D. The purpose is to maximize access to planetary and disk science at separations previously inaccessible in the mid-infrared (Rieke et al., 2015, Boccaletti et al., 2015).

MIRI’s optical train is cooled to <7 K using a hybrid mechanical/Joule-Thomson cooler. This eliminates instrument self-emission, preserves optical alignment, and delivers background-limited sensitivity that would otherwise be impossible at these wavelengths (Wright et al., 2015).

2. PSF Characterization and Resolution Enhancement

MIRI coronagraphic performance is tightly coupled to the PSF’s sharpness and stability. At 5.6 μm, the PSF is undersampled by the detector’s 0.11″ pixel scale, so a microscanning technique—subpixel stepping of a point source in an 11×11 grid—was developed to acquire a set of low-resolution images (Guillard et al., 2010). This approach enables over-resolution via a deconvolution algorithm:

The forward model for each acquired image is

$y_k = S \cdot R \cdot T_k x + n_k$

where $T_k$ is the subpixel translation, $R$ is the detector’s impulse response, $S$ is the downsampling, and $n_k$ is noise.

The high-resolution PSF $x$ is found by minimizing

$Q(x) = \|z - Hx\|^2 + \mu \|D x\|^2$

with $H = AR$ (A masks missing data), $D$ a finite difference operator, and $\mu$ the regularization parameter. The formal solution is

$\hat{x} = (H^T H + \mu D^T D)^{-1} H^T z$

but solved via a conjugate gradient scheme for tractability.

With this microscanning+deconvolution method, the reconstructed 5.6 μm PSF yields a FWHM of 0.18–0.20″ and 56–59% encircled energy within the first dark Airy ring (radius 5″), matching OBA-1004 optical quality requirements and simulations after correcting mirror alignment (Guillard et al., 2010). At longer wavelengths, the energy fraction rises to 57–68%.

Such a precisely constrained PSF is essential for mask optimization, PSF subtraction, and residual starlight calibration in high-contrast imaging.

3. Contrast Performance and Science Capability

Pre-flight and laboratory testing established on-axis rejection factors of >100–300 for the 4QPMs and >800 for the Lyot in ideal conditions (Boccaletti et al., 2015). Simulations incorporating realistic wavefront errors (~130–204 nm rms), pointing jitter (e.g., 7 mas rms/axis), and detector noise yield raw contrast limits of 10^{–4–10^–5} at separations >0.5–1″, improving with reference star subtraction to 2–4×10^–5 (3σ) at 1″ for the best 4QPM filters (Boccaletti et al., 2022).

The practical IWA is defined as the radius for 50% off-axis source transmission (≈1 λ/D for 4QPMs; ≈3.3 λ/D for Lyot), with full transmission at greater separations. These specifications make MIRI’s coronagraphic suite sensitive to planetary companions with T_eff as low as ~400–500 K (Boccaletti et al., 2015 Danielski et al., 2018), and to disk substructures down to 5 AU at 10 pc for the 11–16 μm bands (Rieke et al., 2015).

4. Observing Procedures and Data Processing

MIRI coronagraphic observations require precise target acquisition via small-angle maneuvers, achieving ≤5–10 mas centering accuracy on the mask (Boccaletti et al., 2015 Boccaletti et al., 2022). Filters are selected from the FWA. Reference stars—ideally matched in spectral type and brightness—are observed with identical acquisition sequences for optimal PSF calibration.

Data processing leverages reference differential imaging (RDI) using both classical and PCA-based subtraction algorithms. SpaceKLIP and pyKLIP routines enable robust PSF registration and PSF subtraction (Kammerer et al., 2022). Advanced post-processing, such as small-grid dithering (micro-offsets), further improves the diversity of reference libraries and reduces speckle noise, pushing sensitivity limits closer to the photon and detector noise floors.

PSF fidelity is impacted by several systematics, notably the “brighter-fatter effect” (BFE) arising from electronic charge redistribution in the Si:As IBC detectors. The BFE causes the effective PSF to broaden by 10–25%, with direct relevance for absolute flux calibration and the subtraction of the stellar PSF in coronagraphic exposures (Argyriou et al., 2023). This is addressed by constructing empirical 3×3 pixel deconvolution kernels using electrostatic models of the detector and implementing self-calibration algorithms.

5. Scientific Applications and Early On-Sky Results

MIRI coronagraphy enables the direct detection and photometric/spectral characterization of exoplanets and disks in the mid-infrared, a wavelength regime previously inaccessible at high contrast and spatial resolution.

Key results include:

Detection of exoplanets (e.g., GJ 504 b, HR 8799 b–e, HD 95086 b) down to T_eff ≈ 500–1000 K and R ≈ 1 R_Jup, with mid-IR photometry providing reduced uncertainties and resolving atmospheric degeneracies (e.g., between radius, temperature, metallicity). For GJ 504 b, the F1065C filter data yield a 12.5σ detection of NH₃, with volume mixing ratios consistent with model predictions for planetary-mass objects (Mâlin et al., 30 Dec 2024). The inclusion of MIRI fluxes shifts best-fit radii and temperatures into better agreement with evolutionary models for directly imaged exoplanets (A. et al., 2023, Mâlin et al., 29 Aug 2024).
Resolution of inner debris disks at radii ~10–30 AU in systems such as HR 8799 and HD 95086, and detection of extended cold dust belts—e.g., HD 106906 at R ≃ 70 AU—using mid-IR thermal emission (A. et al., 2023 Rouan et al., 18 Apr 2025).
First mid-infrared detection of a white dwarf companion at ~10⁴ contrast, demonstrating the potential for precise flux and SED measurements of faint objects in the glare of bright primaries (Venner et al., 15 Oct 2024).

Advanced image modeling—including simultaneous disk and planet subtraction, combined with multi-wavelength forward modeling (e.g., DDiT+)—enables detailed structural and grain size constraints for debris disks, complementing sub-mm facilities like ALMA (Rouan et al., 18 Apr 2025).

6. Calibration Strategies and Future Prospects

The precision demanded by MIRI coronagraphic imaging has driven the development of rigorous calibration protocols, extending from laboratory characterization—including PSF microscanning and deconvolution—to in-flight background subtraction, reference library optimization, and self-consistent BFE modeling (1006.57352303.13517).

Custom simulation frameworks, notably MIRISim, support mission planning and data interpretation, leveraging Calibration Data Products (CDPs) for realistic detector and optical modeling (Klaassen et al., 2020). As data reduction pipelines evolve to address systematics such as the BFE and stray light (“glow sticks”), further gains in contrast sensitivity and repeatability are anticipated.

MIRI’s coronagraphs continue to define the standard for high-contrast, mid-infrared imaging. As its capabilities are expanded through continued operational refinements and synergies with other JWST instruments, the prospects for comprehensive exoplanet atmosphere studies, resolved disk investigations, and faint binary companion characterization remain central to mid-infrared astrophysics.

Table: MIRI Coronagraph Key Parameters

Coronagraph	Central λ (µm)	Design	IWA (λ/D)	On-Axis Rejection
F1065C	10.65	4QPM	~1	~260
F1140C	11.40	4QPM	~1	~285
F1550C	15.50	4QPM	~1	~310
F2300C	23.00	Lyot spot	~3.3	~850

On-axis rejection and IWA values reference ground and in-flight test estimates (Boccaletti et al., 2015 Boccaletti et al., 2022).

In summary, JWST/MIRI coronagraphic imaging integrates sophisticated optical engineering, advanced calibration, and powerful data analysis to deliver transformative capabilities for high-contrast mid-infrared astronomy across a broad range of scientific domains.