  Asteroids (162173) Ryugu and (101955) Bennu observed by Hayabusa2 and
OSIRIS-REx share many properties, but spectral observations by the telescopic
Optical Navigation Camera (ONC-T) and MapCam detected subtle but significant
differences, which may reflect differences in their origin and evolution.
Comparing these differences on the same absolute scale is necessary for
understanding their causes. However, ONC-T and MapCam have a large
imager-to-imager systematic error of up to 15% caused by the difference in
radiometric calibration targets. To resolve this problem, we cross calibrated
albedo and color data between the two instruments using the Moon as the common
calibration standard. The images of the Moon taken by ONC-T and MapCam were
compared with those simulated using photometry models developed from lunar
orbiter data. Our results show that the cross-calibrated reflectance of Ryugu
and Bennu can be obtained by upscaling the pre-cross-calibrated reflectance of
Bennu by 13.3 +/- 1.6% at b band, 13.2 +/- 1.5% at v band, 13.6 +/- 1.7% at w
band, and 14.8 +/- 1.8% at x band, while those for Ryugu are kept the same.
These factors compensate for the imager-to-imager bias caused by differences in
targets used for radiometric calibration and solar irradiance models used for
data reduction. Need for such large upscaling underscore the importance of
using the cross-calibrated data for accurately comparing the Ryugu and Bennu
data. The uncertainty in these factors show that the reflectance of Ryugu and
Bennu can be compared with <2% accuracy after applying our results. By applying
our cross calibration, the geometric albedo of Bennu became consistent with
those observed by ground-based telescopes and OVIRS. Our result can be simply
applied by multiplying a constant to the publicly available data and enables
accurate comparison of the optical spectra of Ryugu and Bennu in future
studies.
