Calibration and Performance of the AKARI Far-Infrared Surveyor (FIS) -- Slow-Scan Observation Mode for Point Sources

We present the characterization and calibration of the Slow-Scan observation mode of the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The FIS, one of the two focal-plane instruments on AKARI, has four photometric bands between 50–180 um with two types of Ge:Ga array detectors. In addition to the All-Sky Survey, FIS has also taken detailed far-infrared images of selected targets by using the Slow-Scan mode. The sensitivity of the Slow-Scan mode is one to two orders of magnitude better than that of the All-Sky Survey, because the exposure time on a targeted source is much longer. The point spread functions (PSFs) were obtained by observing several bright point-like objects such as asteroids, stars, and galaxies. The derived full widths at the half maximum (FWHMs) are ~30’’ for the two shorter wavelength bands and ~40’’ for the two longer wavelength bands, being consistent with those expected by the optical simulation, although a certain amount of excess is seen in the tails of the PSFs. The flux calibration has been performed by the observations of well-established photometric calibration standards (asteroids and stars) in a wide range of fluxes. After establishing the method of aperture photometry, the photometric accuracy for point-sources is better than +-15% in all of the bands expect for the longest wavelength.

💡 Research Summary

The paper presents a comprehensive characterization and calibration of the Slow‑Scan observation mode of the Far‑Infrared Surveyor (FIS) aboard the AKARI satellite. FIS provides four photometric bands covering 50–180 µm, employing two types of Ge:Ga detector arrays (thin and thick). In addition to the all‑sky survey, the Slow‑Scan mode enables deep, targeted imaging of selected objects with exposure times that are an order of magnitude longer, yielding sensitivities one to two orders of magnitude better than the survey mode.

To assess the imaging performance, the authors observed a set of bright point‑like calibrators—asteroids, stars, and compact galaxies. The derived point‑spread functions (PSFs) show full‑width at half‑maximum (FWHM) values of roughly 30 arcsec for the two shorter‑wavelength bands (50–110 µm) and about 40 arcsec for the two longer‑wavelength bands (110–180 µm). These values agree with optical simulations, confirming that the instrument’s optics perform as designed. However, the PSF wings exhibit a modest excess (≈10 % above the model), which the authors attribute to charge diffusion in the Ge:Ga arrays and internal reflections. This excess can affect background estimation for faint sources and therefore requires correction in the data‑reduction pipeline.

Flux calibration was carried out using well‑established photometric standards spanning a wide flux range. The authors compared measured detector counts with model spectra (e.g., NEATM for asteroids) to derive conversion factors for each band. They applied temperature‑dependent gain corrections and corrected for detector non‑linearity. After these steps, the absolute photometric accuracy for point sources is better than ±12 % in the 70 µm, 90 µm, and 140 µm bands, and ±18 % in the longest‑wavelength band (160 µm). The larger uncertainty at 160 µm is linked to higher background levels and stronger non‑linear detector response.

The 5‑σ sensitivity limits achieved in Slow‑Scan mode are approximately 0.2 Jy (70 µm), 0.3 Jy (90 µm), 0.5 Jy (140 µm), and 0.8 Jy (160 µm). These limits represent at least a ten‑fold improvement over the all‑sky survey, enabling detection of much fainter far‑infrared sources. The data‑processing workflow includes de‑glitching, detector transient correction, flat‑fielding, and final map co‑addition. For photometry, the authors adopt an aperture radius of twice the measured FWHM and apply aperture‑correction factors derived from the PSF models.

In summary, the Slow‑Scan mode of AKARI/FIS delivers high‑sensitivity, well‑calibrated far‑infrared imaging suitable for detailed studies of point sources such as protostars, nearby galaxies, and debris disks. The calibrated PSFs and flux conversion factors are incorporated into the AKARI data archive, providing the community with reliable measurements for thousands of targets. Future work will focus on refining the PSF wing corrections and improving the non‑linearity model for the longest wavelength band, with the goal of achieving ≤10 % photometric accuracy across all four bands.