JWST NIRCam & MIRI Observations

• JWST NIRCam and MIRI are premier infrared instruments offering high sensitivity and resolution from 0.6 to 28 μm, which are essential for diverse astrophysical investigations.
• Their combined imaging and spectroscopy modes enable optimized exoplanet atmosphere characterization, detailed stellar population studies, and efficient deep-field surveys.
• Innovative observational strategies and advanced data reduction techniques enhance calibration accuracy, facilitating breakthroughs in galaxy evolution and dust mapping.

JWST NIRCAM and MIRI Observations

The James Webb Space Telescope (JWST) is equipped with two principal imaging and spectroscopic instruments in the infrared: the Near-Infrared Camera (NIRCam) and the Mid-Infrared Instrument (MIRI). Together, NIRCam and MIRI deliver unprecedented sensitivity, spatial resolution, and wavelength coverage (0.6 μm to 28 μm), enabling research spanning exoplanet characterization, stellar population analysis, dust production, circumgalactic baryonic cycling, and studies of the high-redshift universe. This article reviews the key technical implementations, scientific advances, and operational considerations associated with JWST NIRCam and MIRI observations.

1. Instrument Architectures and Spectroscopic Modes

NIRCam consists of two channels operating simultaneously: a short-wavelength (SW) channel covering 0.6–2.3 μm and a long-wavelength (LW) channel covering 2.4–5.0 μm, each utilizing adjacent fields and equipped with dichroic splitting. Imaging is the primary mode, but both channels support slitless grism spectroscopy. The LW channel includes dedicated grisms (dispersion 1.0 nm/pixel), delivering $R=1200$—$1550$ over 2.4–5.0 μm with different broadband filters (e.g., F322W2 and F444W). The SW channel includes the Dispersed Hartmann Sensor (DHS), which, when configured with the F150W2 filter, provides simultaneous $R \sim 300$ spectroscopy across $\sim$1.01–2.02 μm, dispersing the spectrum into 10 spatially separated beams and sampling small, non-contiguous fractions of the pupil (~29% throughput accounting for obscurations) (Schlawin et al., 2016).

MIRI operates from 5–28 μm and supports imaging (with wide and medium bands), low-resolution slitless spectroscopy, medium-resolution IFU spectroscopy, and several coronagraphic configurations. The Si:As impurity band conduction (IBC) detector arrays, with unique non-linearity and charge redistribution behavior, define its instrument response characteristics (Argyriou et al., 2023).

2. Observational Strategies and Data Reduction

Optimal use of NIRCam and MIRI frequently entails multi-band or parallel observations. For example, MIRI imaging parallels can be performed during prime NIRCam exposures, as in the MINERVA and COSMOS-Web surveys (Muzzin et al., 25 Jul 2025, Harish et al., 3 Jun 2025). For exoplanet spectroscopy, simultaneous SW and LW NIRCam dispersive readout (i.e., DHS plus LW grism) maximizes information content per observation, minimizing transit or eclipse count and overall time investment (Schlawin et al., 2016, Schlawin et al., 2018).

Data reduction builds on the JWST Calibration Pipeline but nearly all major surveys employ further customizations, such as modified pixel masking (for “warm” or defective pixels), iterative “super background” subtraction to remove residual gradients and banding (especially in MIRI mosaics), tailored flat-field handling (notably for SLOWR1 vs FASTR1 read modes), and astrometric refinements—often using high-density HST catalogs as the frame-opposed to Gaia, due to the small MIRI field of view (Yang et al., 2023, Backhaus et al., 24 Mar 2025).

In cases of extended emission (e.g., circumgalactic dust filaments in NGC 891 or complex star-forming regions in Westerlund 1), background models are built from dithered frames and source-masked mosaics. Accurate estimation of noise in large apertures employs empirically fit, power-law scaling to account for correlated pixels introduced by drizzling or resampling (e.g., $\sigma_N = \sigma_1 \alpha N^\beta$) (Harish et al., 3 Jun 2025).

3. Key Technical and Astrophysical Findings

Exoplanet and Atmosphere Characterization

JWST NIRCam and MIRI enable comprehensive transmission and secondary eclipse spectroscopy for exoplanetary atmospheres. Inclusion of the SW DHS channel in NIRCam grism observations simultaneously covers the 1–2 μm (important H$_2$O absorption), while LW grisms span 2.4–5.0 μm (probing CO, CO$_2$, and CH$_4$). Simultaneous acquisition reduces the need for separate transit events, optimizing observatory use (Schlawin et al., 2016). Synthetic retrievals indicate that metallicity can be constrained to 20–170% and C/O ratio to $\sim$10–60% in favorable cases (Schlawin et al., 2018).

MIRI imaging at 21 μm (F2100W) is shown to be uniquely capable of detecting cold, mature gas giants with $T_{\rm eff} < 100$ K, similar to Jupiter and Saturn, especially for nearest M dwarf systems ($<$3 pc). In contrast, NIRCam coronagraphy at 4.4 μm is limited by cloud opacity in cold atmospheres—the mid-IR is far less affected by atmospheric clouds, making MIRI imaging superior for the direct discovery of “solar system analogs” (Bowens-Rubin et al., 21 May 2025).

Stellar Populations and Cluster Analysis

NIRCam and MIRI observations of stellar environments such as Westerlund 1 reach photometric depths ($m_{F115W} \sim 23.8$ mag at 50% completeness) equivalent to $\sim$0.06 M$_\odot$—well within the brown dwarf regime (Guarcello et al., 20 Nov 2024). Point-source detection and photometry integrate both PSF-fitting and aperture methods, often supported by cross-matching with ancillary data sets (e.g., Chandra X-ray catalogs) using statistically rigorous likelihood ratio criteria: $LR = \frac{q(m)\,f(r)}{n(m)}$ where $q(m)$ is the magnitude probability distribution, $f(r)$ the radial offset probability (Gaussian), and $n(m)$ the field magnitude distribution.

MIRI’s sensitivity and resolution at $\sim$0.25″ facilitate mapping of diffuse emission morphologies—features such as “droplets,” pillars, and outflow shells around massive evolved stars (e.g., W75, W26) are resolved and associated with feedback from stellar winds and past supernovae in supermassive clusters (Guarcello et al., 20 Nov 2024).

Dust Production, Star Formation, and Galaxy Evolution

Multi-band NIRCam and MIRI imaging in nearby galaxies (e.g., NGC 6822, NGC 891) reveal previously undetected populations: deep CMDs constructed in $F115W-F200W$ and $F770W-F2100W$ color space distinguish main sequence, red clump, AGB bumps, CAGB/OAGB, and heavily dust-enshrouded YSOs (Nally et al., 2023, Chastenet et al., 15 Aug 2024). MIRI’s sensitivity enables detection of asymptotic giant branch (AGB) stars responsible for dust production in metal-poor environments, while spatially resolved mid-IR filaments up to several kpc above galaxy disks (e.g., NGC 891) illuminate feedback-driven baryonic cycling (Chastenet et al., 15 Aug 2024).

In deep extragalactic fields, MIRI imaging to 28.65 AB mag enables the paper of rest-frame optical emission from galaxies at $z > 9$. The depths and spatial resolution, in tandem with NIRCam photometry, permit determination of stellar masses, morphologies, and the detection of dusty or high-redshift populations only visible in the mid-IR (Östlin et al., 29 Nov 2024, Muzzin et al., 25 Jul 2025). Specialized medium-band NIRCam surveys such as MINERVA reduce photometric redshift uncertainties (NMAD from 3.4% to 0.9%) and improve stellar mass determinations, critical for precise galaxy evolution analyses (Muzzin et al., 25 Jul 2025).

A representative summary table of imaging parameters and sensitivities observed in recent MIRI surveys is shown below.

Filter/Band	5σ Depth (μJy)	Area (arcmin²)
F770W	0.18–0.2	0.2 deg² (COSMOS-Web)
F1280W	0.65	275
F1500W	1.26	275
F2100W	4.10	275

4. Detector and Calibration Effects

The MIRI detector exhibits a distinct “Brighter-Fatter Effect” (BFE): as pixels accumulate charge, local debiasing shrinks the depletion region, redistributing photocurrent to adjacent pixels and broadening the PSF and emission line profiles by 10–25% relative to expectation for uniform illumination. The intra-pixel electric field geometry must be explicitly modeled (e.g., solving $$\nabla^2 \phi(x, y, z) = \rho(x, y, z)/\epsilon_{\rm Si}$$ in 3D), and BFE effects must be treated in conjunction with non-linearity correction (Argyriou et al., 2023). This is relevant for absolute flux calibration, PSF-weighted photometry, time-series studies (e.g., exoplanet transits), and spectral extraction, requiring the use of tailored deconvolution kernels.

Point-source photometric reproducibility has been benchmarked against Spitzer/IRAC in overlapping bands (e.g., F770W vs. IRAC Ch. 4), with median offsets $<0.05$–$0.1$ mag, validating JWST’s calibration pipeline for flux scale uniformity (Yang et al., 2023, Harish et al., 3 Jun 2025).

5. Advanced Applications and Data Products

NIRCam and MIRI enable empirical classification of sources via machine-learning approaches (e.g., AGNBoost based on XGBoostLSS using NIRCam+MIRI colors). Modeling leverages up to 121 input features (band magnitudes, colors, quadratic color combinations) to predict parameters such as AGN mid-IR power-law fraction (frac$_{\rm AGN}$) and photometric redshift with competitive outlier fractions ($<$0.2% for simulations; $\sim$17% for $z_{\rm spec}>2$ in survey data), and can impute missing photometry via GAN-based algorithms (Hamblin et al., 3 Jun 2025).

Large imaging surveys (e.g., MEGA, COSMOS-Web, MINERVA) provide science-ready mosaics, photometry, and weight maps to the community, ensuring maximal legacy value. Custom background subtraction, careful PSF modeling, and completeness simulations are routine for accurate source extraction and subsequent analyses (e.g., number counts, mass functions).

6. Implications and Scientific Impact

The combination of JWST NIRCam and MIRI observations provides broad, continuous coverage from the near- to mid-infrared with unprecedented depth and angular resolution. These capabilities enable:

• High-precision exoplanet atmosphere characterization and eclipse mapping, including measurement of molecular abundances, hotspot offsets, and constraints on formation (Schlawin et al., 2016, Schlawin et al., 2018, Kammerer et al., 28 May 2024).
• Detailed stellar population studies spanning brown dwarfs to supergiants, accurate tracing of star formation histories, and direct mapping of dust and feedback-driven phenomena in clusters and galaxies (Nally et al., 2023, Guarcello et al., 20 Nov 2024).
• Direct detection and characterization of cold, mature exoplanets beyond current radial-velocity and microlensing capabilities, anchoring the census of solar system analogs in the solar neighborhood (Bowens-Rubin et al., 21 May 2025).
• Population and morphology studies of high-redshift galaxies—enabling robust photometric redshifts, stellar mass, and structural measurements during the epoch of reionization and cosmic noon (Östlin et al., 29 Nov 2024, Muzzin et al., 25 Jul 2025, Wu et al., 30 Jul 2025).
• Machine learning methods that accelerate identification and classification of AGN and provide efficient redshift proxies for large sky surveys (Hamblin et al., 3 Jun 2025).

These advances demonstrate the power of integrating NIRCam and MIRI data, especially when leveraging parallel observations and advanced data reduction/analysis techniques, and they broaden the methodological horizon for studies of resolved and unresolved astrophysical systems throughout cosmic history.