In-orbit focal adjustment of the AKARI telescope with IRC images

TL;DR

This paper reports on the successful in-orbit focal adjustment of the AKARI telescope using IRC images, improving its imaging performance to near-diffraction-limited at 7.3 um, close to laboratory expectations.

Contribution

It demonstrates a practical method for in-orbit focus adjustment of a space telescope using onboard imaging and model analysis.

Findings

Achieved near-diffraction-limited imaging at 7.3 um in orbit.

Successfully adjusted the focus by shifting the secondary mirror twice.

In-orbit performance closely matches laboratory predictions.

Abstract

AKARI currently in space carries onboard a cryogenically-cooled lightweight telescope with silicon carbide mirrors. The wavefront error of the AKARI telescope obtained in laboratory measurements at 9 K showed that expected in-orbit imaging performance was diffraction-limited at a wavelength of 6.2 um. 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 the jettison of the cryostat aperture lid in the orbit, we observed a star with the InfraRed Camera (IRC) onboard AKARI. Since the initial star images observed in the near-infrared (IR) bands were significantly blurred, we twice moved the secondary mirror for the focal adjustment based on the results of model analyses as well as data analyses of the near-IR images. In consequence, we have successfully adjusted the focus of the telescope. The in-orbit imaging performance thus obtained for the AKARI telescope is diffraction-limited at a wavelength of 7.3 um, slightly degraded from that expected from the laboratory measurement.