PRIMAger: Far-Infrared Imaging & Polarimetry

• PRIMAger is a far-infrared camera that integrates hyperspectral imaging and polarimetry, covering 24–264 μm with a cryogenically cooled 1.8 m telescope.
• It features a dual focal-plane design with PHI (R=8 hyperspectral mode) and PPI (four-band polarimetry) to deliver versatile, wide-area mapping capabilities.
• Advanced cryogenic systems and kinetic inductance detectors enable high sensitivity and efficient confusion-limited surveys across both galactic and extragalactic environments.

PRIMAger is the far-infrared imaging instrument of the PRobe far-Infrared Mission for Astrophysics (PRIMA), a mission concept in Phase A built around a 1.8 m telescope actively cooled to 4.5 K. In the dedicated instrument description, PRIMAger is a two-focal-plane camera using ultra-sensitive kinetic inductance detector arrays to provide coverage from 24 to 264 μm, with a hyperspectral mode over 24–84 μm at $R=8$ and a polarimetric mode in four broad bands from 80 to 264 μm (Ciesla et al., 1 Sep 2025). Other PRIMA papers describe the same instrument as covering approximately 25–260 μm or 25–235 μm, which suggests that published science cases use different band-edge conventions (Burgarella et al., 22 Sep 2025, Donnellan et al., 15 Dec 2025).

1. Mission setting and nomenclature

PRIMA is presented in the recent far-infrared literature as a cryogenically cooled observatory carrying two science instruments, PRIMAger and FIRESS, with the majority of observing time intended for community-led General Observer programs; one mission-level description states that 75% of the mission time over 5 years would be available through the GO program (Moullet et al., 2023). Within that architecture, PRIMAger is the imaging, spectrophotometric, and polarimetric instrument, while FIRESS is the spectrometer (Ciesla et al., 1 Sep 2025).

The instrument is being developed by an international collaboration bringing together Laboratoire d'Astrophysique de Marseille and CEA through CNES in France, SRON in the Netherlands, Cardiff University in the United Kingdom, and JPL and GSFC in the United States, with additional involvement from MPIA for the beam steering mirror (Ciesla et al., 1 Sep 2025). Mission-level summaries repeatedly frame PRIMAger as the element that turns PRIMA into a survey observatory spanning the wavelength gap between JWST at shorter wavelengths and ALMA or NOEMA at longer wavelengths (Burgarella et al., 22 Sep 2025, Moullet et al., 14 Nov 2025).

A persistent source of confusion is the acronym itself. In earlier VLTI literature, PRIMA denoted the “Phase-Referenced Imaging and Micro-arcsecond Astrometry” facility, a dual-feed interferometric system for narrow-angle astrometry and phase-referenced imaging equipped with a K-band fringe sensor unit [(Sahlmann et al., 2010); (Sahlmann et al., 2012)]. In the modern far-infrared mission literature, the same acronym denotes the “PRobe far-Infrared Mission for Astrophysics,” and PRIMAger is the associated far-infrared camera (Ciesla et al., 1 Sep 2025).

2. Optical configuration and observing modes

PRIMAger is organized around two focal-plane subsystems: the PRIMA Hyperspectral Imager (PHI) and the PRIMA Polarimetric Imager (PPI). PHI covers 24–84 μm in two sub-bands, PHI1 at 24–45 μm and PHI2 at 45–84 μm, using linear variable filters placed above the detector arrays; the nominal hyperspectral resolving power is $R=\lambda/\Delta\lambda \ge 8$ (Ciesla et al., 1 Sep 2025). PPI covers 80–264 μm in four broad bands with approximately $R \approx 4$, with central wavelengths listed in the instrument paper as 92, 126, 183, and 235 μm; several science-case papers instead use working bands at 96, 126, 172, and 235 μm (Ciesla et al., 1 Sep 2025, Paré et al., 14 Mar 2025).

Mode	Coverage and resolution	Representative beam/FOV
PHI	24–45 μm and 45–84 μm, $R=8$	FWHM $\approx 4.7''$ and $8.7''$; FOV $3.9' \times 1.3'$ and $3.9' \times 1.4'$
PPI	Four broad bands from 80–264 μm, $R \approx 4$	FWHM $10.9'', 14.9'', 21.7'', 27.9''$; each FOV around $R=\lambda/\Delta\lambda \ge 8$0

The two focal planes share the PRIMAger field of view and are separated by 4.7 arcmin on the sky; the overall telescope field available to PRIMAger is $R=\lambda/\Delta\lambda \ge 8$1, while the individual detector arrays are of order $R=\lambda/\Delta\lambda \ge 8$2 (Ciesla et al., 1 Sep 2025). For PPI specifically, the four detector-array footprints are contained within an approximately $R=\lambda/\Delta\lambda \ge 8$3 field, enabling truly simultaneous multi-band mapping (Molinari et al., 16 May 2025).

The optical train is differentiated by mode. PHI uses an $R=\lambda/\Delta\lambda \ge 8$4 path and PPI a common $R=\lambda/\Delta\lambda \ge 8$5 path (Ciesla et al., 1 Sep 2025). PPI pixels are sensitive to one of three linear polarization angles, and the instrument recovers the Stokes parameters $R=\lambda/\Delta\lambda \ge 8$6, $R=\lambda/\Delta\lambda \ge 8$7, and $R=\lambda/\Delta\lambda \ge 8$8; one survey description states that PRIMAger can obtain Stokes $R=\lambda/\Delta\lambda \ge 8$9, $R \approx 4$0, and $R \approx 4$1 in single scans (Ciesla et al., 1 Sep 2025, Burgarella et al., 22 Sep 2025). PHI and PPI are not Nyquist-sampled instantaneously, so both modes require scanning for full image reconstruction (Ciesla et al., 1 Sep 2025).

PRIMAger is therefore a scanning instrument rather than a staring camera. The beam steering mirror (BSM), located at the entrance pupil, provides two-axis steering up to $R \approx 4$2 on the sky with positional accuracy better than $R \approx 4$3 RMS, and can execute Lissajous, boustrophedon, and triangular scan patterns with motions up to $R \approx 4$4 and accelerations up to $R \approx 4$5 (Ciesla et al., 1 Sep 2025). The instrument description explicitly states that there is no staring or snapshot mode (Ciesla et al., 1 Sep 2025).

3. Cryogenic implementation, detectors, and readout

The observatory provides PRIMAger with a 1.8 m telescope at 4.5 K, while the PRIMAger optical bench and many optics are cooled to 1 K and the detector modules operate at 125 mK (Ciesla et al., 1 Sep 2025). The 4.5 K and 1 K stages are supported by a JWST/MIRI-like Joule–Thomson system, and the detector stages use a NASA Goddard continuous adiabatic demagnetization refrigerator (Ciesla et al., 1 Sep 2025). The same cryochain supports PRIMAger and FIRESS, although the two instruments do not operate simultaneously (Ciesla et al., 1 Sep 2025).

Both focal planes use microwave kinetic inductance detectors. In PPI, the pixels are leaky lens-antenna-coupled NbTiN–aluminum devices, each with a polarization-sensitive antenna on a thin membrane coupled to a silicon lens. In PHI, the antenna is replaced by an absorber structure, which eases alignment tolerances between detector and lens array (Ciesla et al., 1 Sep 2025). Prototype devices at 1.5 THz and 12 THz have been reported as background-limited, with white noise over the expected power range, a low-frequency rise below about 0.1 Hz, and a dark NEP of about $R \approx 4$6 (Ciesla et al., 1 Sep 2025).

Warm readout electronics are shared between PRIMAger and FIRESS. For PRIMAger, the KID readout band is 2.6–4.9 GHz, placed in the second Nyquist zone of a 5 Gsps ADC/DAC system (Essinger-Hileman et al., 4 Dec 2025). Each readout chain is designed to multiplex more than 1000 detectors over 2.5 GHz instantaneous bandwidth while consuming around 30 W, with a specific goal of 1008 detectors per chain; fine channelization yields tone placement and recovery precision of approximately 9.54 kHz (Essinger-Hileman et al., 4 Dec 2025). The digital architecture uses SpaceCube Mini v3.0 hardware with a radiation-tolerant Kintex KU060 FPGA, custom high-speed digitizer hardware, and RF filtering and switching between PRIMAger and FIRESS (Essinger-Hileman et al., 4 Dec 2025).

Detector qualification has also been addressed experimentally. A 2025 irradiation campaign at PARTREC exposed SRON KID arrays developed for PRIMAger to 184 MeV protons while maintaining the samples at 120 mK for a 12-hour run, and the campaign report states that the arrays showed no significant radiation-induced degradation at 5.7 krad (Sauvage et al., 25 Apr 2026). This suggests that sub-Kelvin KID qualification for the PRIMA environment is being pursued at the system level rather than left to room-temperature proxy tests (Sauvage et al., 25 Apr 2026).

4. Survey operation and performance envelope

PRIMAger is repeatedly described as a survey instrument, and several papers quantify that role with explicit observing plans. A General Observer wide-field study defines the $R \approx 4$7-IR survey over about 25% of the sky, $R \approx 4$8, using PHI and PPI together to collect data on about $R \approx 4$9 galaxies to $R=8$0; the observing estimate for that program is a scan speed of $R=8$1, 4657 scan legs, and a total exposure time of 2059 hours (Burgarella et al., 22 Sep 2025). In that survey, representative 5$R=8$2 point-source limits are 2.54 mJy at 34.3 μm, 3.44 mJy at 64.5 μm, 0.765 mJy at 92 μm, 1.09 mJy at 126 μm, 1.49 mJy at 172 μm, and 2.58 mJy at 235 μm (Burgarella et al., 22 Sep 2025).

Galactic survey designs exploit a different part of the same performance envelope. PRIMAGAL proposes a four-band polarization survey of the Galactic Plane with $R=8$3, totaling 720 square degrees, executable in about 1200 hours including mapping and overheads (Molinari et al., 16 May 2025). A separate nearby-cloud survey proposes mapping the 160 deg$R=8$4 Herschel Gould Belt Survey area to the cirrus confusion limit in polarized light in 170 hours, yielding 5$R=8$5 polarized-intensity sensitivities of 2.52, 1.94, 1.37, and 1.00 MJy sr$R=8$6 in PPI1 through PPI4 (Pattle et al., 1 Sep 2025).

At the instrument-requirement level, the PRIMAger paper defines a baseline deep-survey point-source statistical uncertainty of $R=8$7 for a 1 deg$R=8$8 survey completed in 1500 hours, and a PPI surface-brightness requirement of $R=8$9 from 92 to 235 μm in 2 hours over 10 arcmin$\approx 4.7''$0, with the effective beam area matched to the 235 μm diffraction beam area of about 600 arcsec$\approx 4.7''$1 (Ciesla et al., 1 Sep 2025). In the polarimetric mode, the same paper states that polarized-flux sensitivity is obtained by multiplying the total-intensity sensitivity by $\approx 4.7''$2 because polarization measurement requires differencing (Ciesla et al., 1 Sep 2025).

The science cases also show how these capabilities map onto physical scales. In the CMZ, PRIMAger beam FWHM values of 11, 15, 20, and 28 arcsec correspond to approximately 0.4–1.1 pc at 8 kpc (Paré et al., 14 Mar 2025). In PRIMAGAL, beam FWHM values of 9″ to 24″ across 90–230 μm resolve scales down to about 0.35–0.4 pc at the farthest targeted distances (Molinari et al., 16 May 2025). In nearby molecular clouds, the proposed cloud-to-core survey quotes linear resolutions of $\approx 4.7''$3 pc, or $\approx 4.7''$4 au, across the PPI bands (Pattle et al., 1 Sep 2025).

5. Scientific domains enabled by PRIMAger

Extragalactic programs use PRIMAger chiefly for broad far-infrared SED sampling, obscured-source selection, and large-area statistics. The $\approx 4.7''$5-IR survey is designed to observe roughly $\approx 4.7''$6 galaxies to $\approx 4.7''$7, while a deep-plus-medium-wide polarimetric program discussed in the same context would detect about 10,000 galaxies for statistical studies of dust polarization, inclination, gas density, turbulence, outflows, starburst activity, AGN presence, and environment (Burgarella et al., 22 Sep 2025). For HST-dark galaxies, PRIMAger is described as providing 12 independent flux measurements from 25–84 μm and four longer-wavelength bands centered at 96, 126, 172, and 235 μm, improving the uncertainty width on $\approx 4.7''$8 from about 0.5 dex without PRIMAger to about 0.1–0.15 dex with PRIMAger bands included; the same paper estimates that a sample of about 50–100 HST-dark galaxies could be observed in roughly 50 hours (Gruppioni et al., 2 Sep 2025).

A related use is the identification of deeply obscured nuclei and AGN. One PRIMA study argues that PRIMAger can measure the deep 9.8 μm silicate absorption feature over $\approx 4.7''$9 to 7 because its wavelength coverage shifts that rest-frame mid-infrared signature into the observed-frame far-infrared (Donnan et al., 14 Mar 2025). That work models PRIMAger as a 25–235 μm instrument with 12 short- or mid-band photometric channels plus four longer-wavelength filters, and uses those data to construct color selections for obscured nuclei and to argue that photometric redshifts with about 10% accuracy are plausible (Donnan et al., 14 Mar 2025).

Cluster and environmental studies use PRIMAger for dust physics that earlier far-infrared surveys could not constrain well. In the Virgo-cluster proposal, PRIMAger covers roughly 25–265 μm and is used to map the same 84 deg$8.7''$0 region surveyed by HeViCS, but with the missing 20–80 μm coverage needed to constrain the warm dust component, break SED-fitting degeneracies, and add far-infrared polarimetry of the cold ISM (Fritz et al., 4 Sep 2025). A related cluster-galaxy paper argues that PRIMAger imaging and polarimetry can detect diffuse dust in ram-pressure stripped tails, estimate dust temperature and gas-to-dust ratios, and quantify turbulent magnetic fields; for one representative tail, the paper quotes a PRIMAger surface-brightness sensitivity of about $8.7''$1 at 235 μm at 5$8.7''$2 in 10 hours over 1 square degree (Boselli et al., 2 Sep 2025).

Galactic programs emphasize magnetic fields. PRIMAGAL uses four-band far-infrared polarization mapping of the Galactic Plane to determine magnetic-field strength and orientation toward several thousand filamentary clouds, with the explicit aim of studying the role of magnetic fields in the formation, evolution, and fragmentation of dense ISM filaments down to a minimum scale of 0.4 pc out to 8 kpc (Molinari et al., 16 May 2025). For the Galactic Center, PRIMAger is framed as a full-polarimetric imager covering 80–261 μm with beam sizes of 11–28 arcsec, enabling multi-spatial and multi-frequency polarimetric observations of the CMZ and thereby addressing the apparent tension between radio indications of a vertical or poloidal field and far-infrared dust-polarimetric indications of a field more aligned with the Galactic plane (Paré et al., 14 Mar 2025).

Nearby-cloud programs use the same polarimetric hardware on smaller scales. The cloud-to-core survey proposes unbiased mapping of star-forming molecular clouds within 0.5 kpc, resolving magnetic-field structure from diffuse cloud outskirts into filaments and dense cores with PPI bands at 96, 126, 172, and 235 μm (Pattle et al., 1 Sep 2025). A striation-focused survey uses PRIMAger bands 3 and 4, at 172 and 235 μm, to search for polarization-angle signatures of magnetohydrodynamic waves in the Polaris Flare, Taurus, and Musca; the paper estimates that the three $8.7''$3 target regions can be mapped to more than five-sigma detection in averaged polarized intensity in about 59 hours (Skalidis et al., 1 Sep 2025).

6. Confusion, deblending, and data-analysis strategy

A central technical issue for PRIMAger extragalactic imaging is confusion noise. The most detailed deblending study introduces XID+stepwise, a Bayesian method that exploits PRIMAger’s hyperspectral imaging by propagating flux constraints sequentially from shorter to longer wavelengths (Donnellan et al., 15 Dec 2025). With Euclid-like prior source positions, that paper reports flux recovery to within 20% down to 0.2–0.7 mJy across 45–84 μm, corresponding to factors of 1.3–3.4 fainter than the confusion limit, and accurate fluxes of 0.9, 2.5, 7.6, and 14.8 mJy at 92, 126, 183, and 235 μm, respectively, corresponding to factors of 3–5 better than the confusion limit (Donnellan et al., 15 Dec 2025).

The same study also considers deeper Euclid-based prior catalogs plus weak ancillary flux priors at 25 μm, reporting improvements up to a factor of $8.7''$4 fainter than the confusion limit at 96 μm (Donnellan et al., 15 Dec 2025). Importantly, it does not treat external catalogs as a strict prerequisite: blind source detection in PRIMAger data followed by XID+ deblending is also reported to enable sensitivity beyond the confusion limits using PRIMAger data alone (Donnellan et al., 15 Dec 2025). For IR-luminous galaxies at $8.7''$5, the paper finds robust detections in more than 98% of cases in 12 of the 16 considered channels, implying dense sampling of the far-infrared SED even for sources several factors below the classical confusion limit (Donnellan et al., 15 Dec 2025).

This emphasis on inference and calibration extends beyond source extraction. The shared PRIMA readout architecture is designed so that readout noise remains sub-dominant to detector noise, with low-frequency drift suppressed such that 1/$8.7''$6 noise remains below the white-noise level above 0.3 Hz after common-mode removal, and with on-board cosmic-ray glitch removal added because telemetry cannot support full-rate downlink for all detectors (Essinger-Hileman et al., 4 Dec 2025). A plausible implication is that PRIMAger performance is being defined not only by optics and detector sensitivity, but also by the quality of scanning reconstruction, prior-informed deblending, and on-board time-stream conditioning.

Taken together, the published instrument and science papers portray PRIMAger as a cryogenic far-infrared camera that combines linear-variable-filter hyperspectral imaging, four-band polarimetry, KID focal planes, and scan-based observing into a single platform optimized for wide-area mapping and confusion-limited analysis. Its defining technical identity is the combination of 24–264 μm coverage, $8.7''$7 hyperspectral sampling below 84 μm, four-band polarimetry above 80 μm, and detector and readout systems engineered for large multiplexing and low background (Ciesla et al., 1 Sep 2025).