Slow-scan performance of the far-infrared surveyor (FIS) onboard AKARI

We present the in-orbit performance of slow-scan observation of the Far-Infrared Surveyor (FIS) onboard the AKARI satellite. The FIS, one of the two focal-plane instruments of AKARI, has four photometric bands from 50-180 μm with two kinds of Ge:Ga array detectors. In addition to the All-Sky Survey, the FIS also took images of specific targets by the slow-scan. Because of the longer exposure time on a targeted source, the sensitivity in the slow-scan mode is 1-2 orders of magnitude better than that in the All-Sky Survey mode. In order to evaluate the point spread functions (PSFs), several bright point-like objects such as asteroids, stars, and galaxies were observed. Though significant enhancements are seen at the tails of the PSFs, the derived full width at the half maximum (FWHM) are consistent with those expected from the optical simulation and the laboratory measurements; ∼40″ for two shorter wavelength bands and ∼60″ for two longer wavelength bands, respectively. The absolute photometric calibration has been performed by observing well established photometric calibration standards (asteroids and stars) in a wide range of fluxes. After the establishment for the method of the aperture photometry, the photometric accuracy for point sources is less than 10% in all bands.