  The Optical Navigation Camera (ONC-T, ONC-W1, ONC-W2) onboard Hayabusa2 are
also being used for scientific observations of the mission target, C-complex
asteroid 162173 Ryugu. Science observations and analyses require rigorous
instrument calibration. In order to meet this requirement, we have conducted
extensive inflight observations during the 3.5 years of cruise after the launch
of Hayabusa2 on 3 December 2014. In addition to the first inflight calibrations
by Suzuki et al. (2018), we conducted an additional series of calibrations,
including read-out smear, electronic-interference noise, bias, dark current,
hot pixels, sensitivity, linearity, flat-field, and stray light measurements
for the ONC. Moreover, the calibrations, especially flat-fields and
sensitivities, of ONC-W1 and -W2 are updated for the analysis of the
low-altitude (i.e., high-resolution) observations, such as the gravity
measurement, touchdowns, and the descents for MASCOT and MINERVA-II payload
releases. The radiometric calibration for ONC-T is also updated in this study
based on star and Moon observations. Our updated inflight sensitivity
measurements suggest the accuracy of the absolute radiometric calibration
contains less than 1.8% error for the ul-, b-, v-, Na-, w-, and x-bands based
on star calibration observations and ~5% for the p-band based on lunar
calibration observations. The radiance spectra of the Moon, Jupiter, and Saturn
from the ONC-T show good agreement with the spacecraft-based observations of
the Moon from SP/SELENE and WAC/LROC and with ground-based telescopic
observations for Jupiter and Saturn.
