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

Abstract

Abstract 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$\mu$m 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 $\sim$30$^{\prime\prime}$ for the two shorter wavelength bands and $\sim$40$^{\prime\prime}$ for the two longer wavelength bands, being consistent with those expected by optical simulation, although a certain amount of excess is seen in the tails of the PSFs. A flux calibration was performed by observations of well-established photometric calibration standards (asteroids and stars) over a wide range of fluxes. After establishing the method of aperture photometry, the photometric accuracy for point-sources is better than $\pm$15% in all of the bands, expect for the longest wavelength.