In-orbit focal adjustment of the AKARI telescope with Infrared Camera (IRC) images

AKARI, currently in space, carries onboard a cryogenically cooled light-weight telescope with silicon carbide mirrors. The wavefront error of the AKARI telescope, obtained in laboratory measurements at 9 K, showed that the expected in-orbit imaging performance was diffraction-limited at a wavelength of 6.2 μm. The AKARI telescope has a function of focus adjustment by shifting the secondary mirror in parallel to the optical axis. On the 4th day after jettison of the cryostat aperture lid in orbit, we observed a star with the Infrared Camera (IRC) onboard AKARI. Since the initial star images observed in the near-infrared (NIR) bands were significantly blurred, we twice moved the secondary mirror for a focal adjustment based on the results of model analyses as well as data analyses of NIR band images. As a consequence, we successfully adjusted the focus of the telescope. The thus-obtained in-orbit imaging performance for the AKARI telescope is diffraction-limited at a wavelength of 7.3 μm, slightly degraded from that expected from laboratory measurements.