  Hayabusa2, a Japanese sample-return mission to a C-type asteroid, arrived at
its target 162173 Ryugu in June 2018. The optical navigation cameras (ONC-T,
ONC-W1, ONC-W2) successfully obtained numerous images of Ryugu. ONC-T is a
telescopic framing camera with a charge-coupled device (CCD), has seven filter
bands in ultraviolet, visible and near infrared wavelength ranges, and were
used to map the spectral distribution of the Ryugu surface. Since the locations
of a target seen in ONC-T images are slightly different among different
wavelength images in one multi-band observation sequence due to changes in
spacecraft positions and attitudes during the filter-changing sequence, one of
the image processing issues is image co-registration among images for different
wavelength bands. To quickly complete the image co-registration to meet a
limited mission schedule, we combined conventional image co-registration
techniques with several improvements based on previous planetary missions. The
results of our analysis using actual ONC-T images indicate that image
co-registration can reach accuracy on the order of 0.1 pixels, which is
sufficient for many spectral mapping applications for Ryugu analyses.
