In-orbit focal adjustment of the AKARI telescope with and without liquid helium cryogen

Details In-orbit focal adjustment of the AKARI telescope with and without liquid helium cryogen In-orbit focal adjustment of the AKARI telescope with and without liquid helium cryogen

Aug 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008spie.7010e..87o&link_type=abstract

Space Telescopes and Instrumentation 2008: Optical, Infrared, and Millimeter. Edited by Oschmann, Jacobus M., Jr.; de Graauw, M

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Scientific paper

The AKARI satellite carries a cryogenically cooled telescope of an F/6 Richey-Chetien system with a sandwich-type silicon carbide (SiC) primary mirror of 685mm in effective diameter. The AKARI satellite ran out of the liquid helium (LHe) cryogen on 26 August 2006. With LHe the telescope system was kept around 6K, whereas it is kept around 40K by the on-board cryocoolers after the LHe exhaustion. The telescope system has a focus adjustment mechanism in the secondary mirror assembly. The telescope focus on orbit was adjusted referring to images taken with the Infrared Camera (IRC) on board. The focus adjustment was made both at 6 and 40K. The in-orbit imaging performance at 6K was estimated to be diffraction limited at 7.3μm, a little worse than the laboratory measurements prior to the launch. It was slightly degraded to be approximately diffraction limited at 8μm at 40K as expected from the laboratory test, but the movement of the focus position was in the opposite sense to the ground test. The AKARI mission provided us data of the focus shift with temperature on orbit for the first time. We report an overview of the AKARI telescope system and the focus adjustment operations at 6 and 40K.

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