FIRESS: Far-Infrared Enhanced Survey Spectrometer

• FIRESS is the Far-Infrared Enhanced Survey Spectrometer, offering 24–235 µm coverage with multi-mode, low- to high-resolution capabilities.
• It employs four slit-fed grating modules with KID arrays to capture simultaneous multi-band spectra, achieving resolutions from ~85 up to 20,500.
• FIRESS is key to diagnosing astrophysical processes in protoplanetary disks, interstellar solids, and galaxy evolution within the PRIMA mission framework.

FIRESS, the Far-Infrared Enhanced Survey Spectrometer, is the spectroscopic instrument of PRIMA and is described across the PRIMA literature as the facility’s dedicated far-infrared capability for wide-band line spectroscopy, spectral mapping, and selected high-resolution follow-up. In the mission architecture, FIRESS is the spectroscopic counterpart to PRIMAger’s imaging and polarimetric modes, and it is repeatedly presented as the component that converts PRIMA from a continuum mapper into a physically diagnostic observatory for obscured gas, dust, ices, minerals, protoplanetary disks, and galaxies across cosmic time (Pontoppidan et al., 1 Sep 2025, M. et al., 2 Sep 2025).

1. Mission context and institutional role

PRIMA is described as a cryogenically cooled far-infrared observatory with a 4.5-K, 1.8-m telescope. In mission-level descriptions, the observatory carries two science instruments: PRIMAger, the imaging and polarimetric instrument, and FIRESS, the spectroscopic instrument. The overall PRIMA wavelength domain is given as approximately 24–264 $\mu$m or 25–264 $\mu$m depending on the mission paper, whereas FIRESS-specific papers describe the spectrometer itself as covering 24–235 $\mu$m (Burgarella et al., 22 Sep 2025, Moullet et al., 14 Nov 2025).

FIRESS is not treated as a narrow-purpose add-on. The science-driver paper presents it as a highly versatile, multi-mode spectrometer intended to support three primary science themes—origins of planetary atmospheres, co-evolution of galaxies and supermassive black holes, and buildup of heavy elements and dust in the Universe—while also enabling a broad General Observer program (Pontoppidan et al., 1 Sep 2025).

The mission planning documents make the community role explicit. About 75% of PRIMA observing time is allocated to the General Observer program, and the GO science book is described as already containing 76 science cases, about two-thirds of which use FIRESS either alone or together with PRIMAger. A separate GO-survey paper further notes that, within one volume of proposed programs, about one-third use PRIMAger only and another third combine PRIMAger and FIRESS, indicating that FIRESS is structurally embedded in the mission’s open-use scientific model rather than confined to a small core program (Pontoppidan et al., 1 Sep 2025, Burgarella et al., 22 Sep 2025).

2. Optical architecture and observing modes

The reference FIRESS design spans 24–235 $\mu$m with four slit-fed grating modules operating at low resolution. These modules are logarithmically spaced in wavelength and are coupled to kinetic inductance detector (KID) arrays, with all four focal planes read out simultaneously (M. et al., 2 Sep 2025).

Band	Wavelength range	Focal-plane format
Band 1	24–43 $\mu$m	$24 \times 84$ pixels
Band 2	42–76 $\mu$m	$24 \times 84$ pixels
Band 3	74–134 $\mu$m	$24 \times 84$ pixels
Band 4	130–235 $\mu$0m	$\mu$1 pixels

The low-resolution architecture is built around four R $\mu$2 grating spectrometers, with more detailed papers giving a practical range of roughly $\mu$3–150 depending on band. The science-driver paper organizes FIRESS into three basic observing modes: low-resolution mapping spectroscopy, low-resolution point-source spectroscopy, and high-resolution point-source spectroscopy. It also states that the full 24–235 $\mu$4m range can be covered in only two spectral settings, a design choice that directly serves survey efficiency and broad spectral grasp (Pontoppidan et al., 1 Sep 2025, M. et al., 2 Sep 2025).

A defining architectural feature is the slit pairing on the sky. Bands 1 and 3 are coaligned through a dichroic, and bands 2 and 4 are coaligned at a different field position. As a consequence, a point source can be placed on either bands 1+3 or bands 2+4 simultaneously, so two of the four bands are on-source at a given time. The focal planes are built as 84 (spectral) $\mu$5 24 (spatial) KID arrays, implemented as two 12 $\mu$6 84 subarrays in a hex-packed geometry with 900 $\mu$7m pitch (M. et al., 2 Sep 2025).

The high-resolution capability is provided by a Fourier Transform Module (FTM) inserted upstream of the grating modules. The FTM is described as a polarizing Martin–Puplett interferometer that intercepts the beam, scans optical path difference, and reinserts the processed light into the grating spectrometers for detection. Mission-level summaries describe FIRESS as spanning approximately $\mu$8 up to about 13,000, while the science-driver paper gives the high-resolution mode more specifically as $\mu$9, corresponding to $\mu$0 at 112 $\mu$1m, $\mu$2 at 24 $\mu$3m, and $\mu$4 at 235 $\mu$5m (Moullet et al., 14 Nov 2025, Pontoppidan et al., 1 Sep 2025).

3. Sensitivity, calibration, and operational constraints

For low-resolution pointed spectroscopy, the approach-and-performance paper gives the recommended unresolved-line metric as

$\mu$6

This value is used as a durable planning guideline across the full FIRESS band for unresolved lines. For mapping, the same paper recommends a fiducial exposure to reach $\mu$7 over 100 square arcminutes, with 800 h for $\mu$8 and

$\mu$9

for $\mu$0, with time scaling as area over depth squared (M. et al., 2 Sep 2025).

The instrumental requirements are science-dependent, and several papers emphasize that calibration can be as important as raw sensitivity. The missing-interstellar-oxygen study argues that detecting broad crystalline water-ice bands at 44 and 62 $\mu$1m requires only low spectral resolution, with $\mu$2 sufficient, but demands relative calibration better than 1% because the expected feature contrast is only at the percent level against a strong continuum. In the examples discussed there, the excess above continuum is about 1% in one case and 0.4% in another, implying $\mu$3 and in some cases $\mu$4 (Onaka et al., 2 Sep 2025).

The dust-stripping science case places these performance numbers in a mapped, diffuse-medium context. Adopting a nominal FIRESS line sensitivity of $\mu$5 at 5$\mu$6 over 100 arcmin$\mu$7 in 800 hours for 24–75 $\mu$8m, and the wavelength-dependent scaling above 75 $\mu$9m, it estimates that diffuse stripped tails can be accessible for the brightest cooling lines, while other lines become practical mainly in denser substructures such as compact star-forming clumps (Boselli et al., 2 Sep 2025).

A plausible implication is that FIRESS performance should be understood less as a single sensitivity number than as a combination of band coverage, mapping speed, low-resolution multiplexed discovery spectroscopy, and, in particular cases, continuum fidelity and relative calibration stability.

4. Detectors, focal planes, and supporting technology

FIRESS is based on large-format KID/MKID focal planes and associated multiplexed readout electronics. The readout paper states that the FIRESS detector band is 0.4–2.4 GHz, placed in the first Nyquist zone of a 5 Gsps direct-sampling architecture, whereas PRIMAger occupies 2.6–4.9 GHz in the second Nyquist zone. Each readout chain must handle 1008 detectors for FIRESS across 2.5 GHz instantaneous bandwidth, while consuming around 30 W per readout chain. The fine channelization provides 9.54 kHz tone-placement and recovery precision, satisfying the requirement imposed by resonators with $\mu$0 (Essinger-Hileman et al., 4 Dec 2025).

At full instrument scale, the same paper identifies a downlink-driven need for onboard processing because the spacecraft cannot return native-sampled time-ordered data for all 8064 FIRESS detectors at the native 9.54 kHz rate. This motivates onboard cosmic-ray glitch removal before downsampling to science rates of about 100–700 Hz, depending on observing mode (Essinger-Hileman et al., 4 Dec 2025).

Radiation tolerance at Sun–Earth L2 is treated as an instrument-qualification issue. A cold irradiation study models the expected proton environment and reports a 5.3-year total mission displacement-damage dose of $\mu$1, dominated by solar protons. An aluminum KID array fabricated for FIRESS was irradiated to approximately 62% of this dose. The reported mean quasiparticle lifetime changed from 0.37 ms to 0.36 ms, the mean shift in internal quality factor was positive, and the expected full-dose resonance-frequency scatter remained far below the designed resonator spacing, supporting the view that the tested arrays are compatible with L2 total-dose requirements (Kane et al., 30 Apr 2026).

Optical coupling is provided by monolithic silicon lenslet arrays. The lenslet paper describes 1008-pixel, hexagonally packed arrays at 900 $\mu$2m pitch, fabricated by grayscale lithography followed by deep reactive ion etching, anti-reflection coated with quarter-wavelength Parylene-C, and aligned to the KID arrays using a flip-chip bonder. The achieved post-bond alignment is 3 $\mu$3m, well within the $\mu$4m tolerance, and redesigned hexagonal-corner lenslets deliver about 14% more optical power to the detectors than earlier circular-profile designs (Dahal et al., 13 Nov 2025).

5. Planetary systems, interstellar solids, and depletion reservoirs

One of FIRESS’s flagship roles is in the far-infrared spectroscopy of protoplanetary disks and related solid-state reservoirs. The science-driver paper identifies disk [C/H], [O/H], and C/O as central quantities, and links FIRESS capability to measurements of many water vapor lines, the HD ground-state line at 112 $\mu$5m as a disk gas-mass tracer, the 43 $\mu$6m water ice band, the 69 $\mu$7m forsterite band, and other volatile tracers across the far-infrared. It further states that the HD line-to-continuum ratio should exceed 2.5% for all disks more massive than 1 $\mu$8, with the model-based requirement translating to $\mu$9 for that application (Pontoppidan et al., 1 Sep 2025).

The missing-oxygen study presents FIRESS as the enabling instrument for a specific depletion problem: unidentified depleted oxygen in the translucent and dense interstellar medium. That paper states that at least about a quarter of the total oxygen, roughly $24 \times 84$0 ppm relative to hydrogen, is not accounted for in known reservoirs, and argues that large crystalline water-ice grains are the only plausible oxygen-bearing solid capable of storing large amounts of oxygen without violating abundance constraints. It further shows that the commonly used 3 $24 \times 84$1m ice absorption band becomes unreliable for grains larger than about 3 $24 \times 84$2m, whereas crystalline-ice lattice bands at 44, 52, and 62 $24 \times 84$3m remain diagnostic up to about $24 \times 84$4 and can remain recognizable even at 10 $24 \times 84$5m. In that framing, FIRESS is the instrument class required to test whether large water-ice grains hide a substantial oxygen reservoir (Onaka et al., 2 Sep 2025).

A parallel depletion problem concerns sulfur. The metal-sulfides paper argues that MgS and FeS are plausible refractory sulfur reservoirs in dense interstellar dust and proposes absorption spectroscopy with the FIRESS low-resolution mode against bright protostellar continua. FIRESS is described there as a spectrometer with four long-slit grating modules, 24–235 $24 \times 84$6m coverage, and $24 \times 84$7 in low-resolution mode. Under the paper’s assumptions, the FeS and MgS bands between 20 and 50 $24 \times 84$8m could be detected in absorption with S/N $24 \times 84$9 in 1 h for sources brighter than about 200 mJy, and the study explicitly favors low resolution because a corresponding high-resolution observation at 30 $\mu$0m would require about 74 hours for a 5$\mu$1 detection (Jiménez-Serra et al., 2 Sep 2025).

6. Galaxy evolution, surveys, and combined-instrument science

In extragalactic astronomy, FIRESS is repeatedly presented as the route to a dust-unbiased census of star formation, black-hole accretion, chemical enrichment, and feedback. The cosmic-noon survey paper proposes a 200 arcmin$\mu$2 blind spectroscopic survey with total exposure time of 640 h or about 750 h, depending on field-coverage assumptions, and predicts roughly $\mu$3–900 detectable galaxies in conservative scenarios, or about $\mu$4 detections at $\mu$5 in the 11.3 $\mu$6m PAH band and/or [O III] 52 $\mu$7m line. In that paper, [O IV] 25.9 $\mu$8m is identified as the brightest IR line tracing genuine AGN activity, while the N3O3 combination of [O III] 52,88 $\mu$9m and [N III] 57 $24 \times 84$0m is presented as the most reliable far-infrared tracer of N/O abundance (Fernández-Ontiveros et al., 8 Sep 2025).

The same extragalactic literature positions FIRESS beyond cosmic noon. The PAH paper models low-resolution FIRESS spectroscopy at high redshift and argues that PRIMA observations of PAH emission are $24 \times 84$1 times more efficient than VLA CO(1–0) observations for galaxies with the same infrared luminosity. It further states that FIRESS can detect PAH emission from galaxies with $24 \times 84$2 up to the end of reionization, and possibly beyond for $24 \times 84$3, while also functioning as a PAH mapping instrument for redshift confirmation and star-formation measurements in protoclusters (Yoon et al., 2 Sep 2025).

A related application is the study of HST-dark galaxies, massive dusty systems at $24 \times 84$4–7 that are undetected even in the deepest HST imaging. The HST-dark science case argues that FIRESS spectroscopy is required to determine whether their obscured power source is star formation, AGN, or a mixture, using lines such as [Ne V] 14.3/24.3 $24 \times 84$5m, [O IV] 25.9 $24 \times 84$6m, [Ne II] 12.8 $24 \times 84$7m, [Si II] 34.5 $24 \times 84$8m, [O I] 63 $24 \times 84$9m, and [O III] 52/88 $\mu$0m. That paper estimates that the $\mu$1–25 brightest HST-dark galaxies, typically with $\mu$2 and $\mu$3, can be observed in reasonable time, often about $\mu$4 h per source under a nominal line sensitivity of $\mu$5 for 5$\mu$6 in 1 h (Gruppioni et al., 2 Sep 2025).

FIRESS is also repeatedly framed as complementary to PRIMAger. In ram-pressure stripped cluster tails, PRIMAger imaging locates the dust continuum, PRIMAger polarimetry constrains turbulent magnetic fields, and FIRESS spectroscopy provides the far-infrared cooling lines—especially [C II] 158 $\mu$7m, [O I] 63 $\mu$8m, [N II] 122 $\mu$9m, and [N II] 205 $24 \times 84$0m—needed to infer gas phase, electron density, metallicity, and photoelectric heating efficiency. This complementarity encapsulates the broader PRIMA model: PRIMAger finds and maps the obscured structures, while FIRESS measures the line diagnostics that establish their physical state (Boselli et al., 2 Sep 2025).

In sum, FIRESS occupies the role of PRIMA’s spectroscopic backbone. Its defining characteristics are broad 24–235 $24 \times 84$1m coverage, simultaneous four-band KID-based low-resolution spectroscopy, a Fourier-transform high-resolution mode, and integration into a mission structure in which community use is dominant. The collected science cases suggest a common logic: far-infrared continuum information identifies obscured systems, but the decisive constraints on composition, heating, cooling, ionization, metallicity, depletion, and feedback come from the spectroscopic diagnostics that FIRESS is designed to provide (Moullet et al., 14 Nov 2025, Burgarella et al., 22 Sep 2025).