PRIMAger Polarimetry Imager Overview

• PRIMAger Polarimetry Imager is a spaceborne instrument that performs hyperspectral and polarimetric imaging in the 80–264 μm range to study dust emission and magnetic fields.
• It utilizes lens-antenna coupled hybrid kinetic inductance detectors arranged in three discrete linear polarization orientations to simultaneously measure the Stokes I, Q, and U parameters with high calibration precision.
• Its advanced scanning, deconfusion, and destriping strategies enable large-area mapping with sub-arcminute resolution, significantly enhancing studies of galactic magnetism and extragalactic sources.

The PRIMAger Polarimetry Imager is a spaceborne instrument developed for the PRIMA mission, designed to provide high-sensitivity, large-area, far-infrared polarimetric imaging. Capable of both continuum hyperspectral imaging and precision polarimetric mapping, PRIMAger advances galaxy, ISM, and star formation studies by offering collective measurements of total and polarized dust emission in the 80–264 μm range at sub-arcminute resolution. The imager leverages kinetic inductance detector (KID) arrays with multi-angle polarization sensitivity and incorporates optimized scan and calibration strategies to overcome key astrophysical challenges such as extragalactic source confusion and beam depolarization.

1. Instrument Architecture and Polarimetric Design

PRIMAger consists of dual focal planes supporting "Hyperspectral" (PHI) and "Polarimetric" (PPI) imaging modes (Ciesla et al., 1 Sep 2025). The PPI focal plane is built from lens-antenna–coupled hybrid KIDs, arrayed such that each pixel is maximally sensitive to one of three discrete linear polarization orientations. These orientations, offset by 120°, facilitate simultaneous measurement of Stokes I, Q, and U parameters across the observed sky.

The optical system matches beam sizes to physical pixel sizes (scaling with the observing wavelength) and is optimized for an f/12 input, yielding near-diffraction-limited resolution over a $\sim$11′×11′ field of view. The instantaneous spatial sampling is Fλ (undersampled relative to Nyquist), mandating coordinated raster scanning and beam steering to ensure complete polarization coverage.

Data from the three pixel orientations is linearly combined:

$$\begin{pmatrix} I \ Q \ U \end{pmatrix} = \mathbf{A} \cdot \begin{pmatrix} p_1 \ p_2 \ p_3 \end{pmatrix}$$

where $\mathbf{A}$ contains the orientation and calibration coefficients for the three polarization pixel types (Ciesla et al., 1 Sep 2025), and $p_n$ are the measured signals.

2. Measurement Principles and Calibration

The PPI exploits a three-angle detector geometry rather than rotating half-wave plates, enabling single-exposure linear polarization measurements with minimal systematic drift and higher scan efficiency (Dowell et al., 25 Apr 2024). For each pixel, the detector output is modeled as:

$$S_{j,k} = g (I_p + \varepsilon [Q_p \cos 2(\theta - \phi_j) + U_p \sin 2(\theta - \phi_j)]) + b_j + n_{j,k}$$

where $g$ is the gain, $\varepsilon$ the polarization efficiency ($\sim$0.99), $\theta$ the pixel’s fixed orientation, $\phi_j$ the instrument rotation, $b_j$ a scan-dependent baseline, and $n_{j,k}$ noise (white + $1/f$). Relative gain calibration is mandatory and typically achieved to better than 0.7%, using repetitive mapping at distinct instrument rotation angles and internal calibration sources. Baseline offsets are estimated and subtracted per scan, and data from all polarization orientations are combined via least squares to solve for (I, Q, U).

Crosslinked scanning and destriping, as developed for Herschel and Planck, are essential to mitigate $1/f$ noise and baseline drift. The mapping pipeline applies beam matching and pointing corrections to further reduce artificial polarization signals.

3. Performance, Confusion, and Survey Depth

Far-infrared extragalactic and Galactic plane maps are fundamentally constrained by source confusion, which arises from limited angular resolution and background source blending. In total intensity, the classical confusion limit ($\sigma_\mathrm{conf}$) increases with wavelength as:

$$\sigma_\mathrm{conf}^2 = \int_\text{beam} b^2(\theta, \phi) d\Omega \int_0^{S_\mathrm{lim}} S^2 \frac{dN}{dS} dS$$

where $b$ is the beam response and $dN/dS$ the differential source counts (Béthermin et al., 5 Apr 2024).

Polarimetric confusion is notably lower—by over two orders of magnitude—because Stokes Q and U contributions from misaligned sources tend to cancel. At 235 μm, the confusion limit in polarization is $>$100$\times$ fainter than in intensity; the measured polarization within a beam is typically dominated by the brightest source, with negligible bias from faint blends. Beam clustering affects intensity confusion noise by up to 25%, but has minimal impact in polarization.

Sparse aperture extraction, using 5$\sigma_\mathrm{conf}$ thresholds on 5×5 pixel regions, yields completeness of 50–80% in intensity and high purity in polarization, sufficient to detect $\sim$8,000 polarized sources up to $z=2.5$ for conservative sensitivity. At shorter wavelengths, intensity maps can reach the “knee” in the galaxy luminosity function up to $z\approx3$ and $10^{11} M_\odot$ main-sequence galaxies up to $z\approx5$.

4. Science Drivers: Galactic and Extragalactic Magnetism

PRIMAger is tailored for magnetic field studies across diverse environments (Molinari et al., 16 May 2025, Pattle et al., 1 Sep 2025, Maglione et al., 2 Sep 2025). In star-forming regions, Galactic Plane filaments, and Giant Molecular Clouds, simultaneous four-band polarimetry at 92, 126, 183, and 235 μm resolves the plane-of-sky field orientation and allows statistical measurement of field strength via techniques like Davis-Chandrasekhar-Fermi:

$$B \approx Q \sqrt{4\pi \rho}\; \frac{\delta v}{\delta \theta}$$

where $\rho$ is the mass density, $\delta v$ the velocity dispersion, and $\delta\theta$ the angular dispersion in polarization (Paré et al., 14 Mar 2025). PRIMAger’s sensitivity enables detection of $\sim$1 MJy/sr polarized emission (5$\sigma$) even in diffuse cirrus and robust mapping of field geometries in filaments down to $0.4$ pc at 8 kpc (Molinari et al., 16 May 2025). For molecular cloud mapping, linear scales as small as $10^3$–$10^4$ AU are reached, with mapping speeds 10,000$\times$ faster than airborne instruments.

In extragalactic settings, PRIMAger accesses polarimetric diagnostics of field alignment, turbulence, and magnetization at $\sim$10–20 pc scales for galaxies up to 0.5 Mpc away. Beam depolarization—a key limitation for SOFIA—is minimized by PRIMAger’s finer angular resolution, allowing recovery of intrinsic dispersion-polarization relations:

$P = \frac{\sqrt{Q^2 + U^2}}{I}$

and alignment parameters such as $$\zeta = \cos\!\big(2\,\Delta\theta_\mathrm{spiral}\big)$$. Typical uncertainties in field orientation are $6^\circ$, versus $11^\circ$ for previous FIR missions (Maglione et al., 2 Sep 2025).

5. Synergy Across Bands and Survey Modes

PRIMAger’s dual hyperspectral and polarimetric imaging allows coordinated construction of well-sampled SEDs with polarization diagnostics over 24–264 μm. Deep intensity imaging at shorter wavelengths (PHI bands) identifies sources whose polarization properties are measured in PPI bands. This synergy is central to deblending: intensity priors from higher-resolution PHI images are leveraged to disentangle sources in confusion-limited long-wavelength or polarimetric images (Donnellan et al., 10 Apr 2024).

In large area surveys (e.g., PRIMAGAL), wide-field mapping strategies capture filamentary cloud populations for statistical magnetic field studies, using spatial filtering, high-pass analysis, and correlation with ancillary datasets (Hi-GAL, FUGIN, SEDIGISM). Magnetic field orientation, dispersion, and energetics are linked to ISM structure, star formation rates, and environmental drivers.

For clusters (e.g., Virgo), PRIMAger’s expanded spectral and polarimetric coverage fills gaps left by earlier surveys (HeViCS), improves dust model constraints via spatially resolved SED fits, and enables measurement of intra-cluster magnetic fields and dust polarization.

6. Limitations, Systematics, and Calibration Strategies

Despite its capabilities, PRIMAger faces several systematic and astrophysical limitations:

• Line-of-sight confusion in the Galactic Plane requires advanced filtering and multiwavelength cross-correlation to disentangle overlapping structures.
• Extended diffuse emission can mask filamentary/cellular polarization; spatial filtering and structure-function analysis are employed, but may risk signal loss or distortion.
• Relative gain calibration must achieve $<$0.7% precision; internal lamp monitoring and repetitive angle mapping are core mitigations.
• Beam matching is critical to avoid artificial polarization, with pointing uncertainties kept below $0.06''$ at PPI1 wavelengths and uniform beam shapes across array orientations.
• 1/f noise and detector yield losses are controlled by destriping and scan crosslinking, shown in simulations to limit noise increase to $\sim$30–40% over white noise predictions (Dowell et al., 25 Apr 2024).

Instrument performance is dictated by the noise equivalent intensity (NEI), optical throughput, and integration time, with $\sigma \propto 1/\sqrt{t_\mathrm{obs}}$. Achieving 1 MJy/sr at $5\sigma$ detection in 1.7 hr/deg² for polarization mapping forms the practical sensitivity basis for large surveys.

7. Collaborative Development and Future Prospects

PRIMAger is developed through coordinated collaboration among French (LAM, CEA/CNRS/CNES), Dutch (SRON), UK (Cardiff), and US (JPL, GSFC) institutes (Ciesla et al., 1 Sep 2025). These partnerships extend from detector and cryogenic design to survey strategy, calibration, and data pipeline development.

Planned and active surveys include PRIMA Vista (extragalactic magnetism), PRIMAGAL (Galactic Plane), molecular cloud mapping, and cluster studies such as the Virgo survey; each leverages the instrument’s sub-arcminute resolution and sensitivity over hundreds of square degrees. PRIMAger’s technical depth—complemented by advanced detectors, calibration systems, and scanning algorithms—positions it as a cornerstone for future FIR polarimetric science.

A plausible implication is that PRIMAger’s mapping speed, survey volume, and polarimetric depth enable transformative advances in the empirical and statistical paper of interstellar magnetic fields, dust composition, and the interactions between turbulence, gravity, and magnetism in star formation across the local universe and beyond.