Cross-calibration of the Rosetta Navigation Camera based on images of the 67P comet nucleus

Abstract

The Rosetta spacecraft carried a Navigation Camera (NavCam) for optical navigation in the vicinity of the comet. In order to facilitate the use of the data for quantitative scientific work, we performed a cross-calibration study based on images taken with the OSIRIS near-angle camera. For this purpose, we selected sets of images acquired roughly simultaneously on 2014 August 1 during comet approach at small phase angles. We employed two procedures, the first one based on the average signal over the nucleus and the second considering histograms of signal values within the images. Both methods delivered consistent results for the radiometric calibration factor. As a first application and further consistency check, we employed the calibration procedure to an extended set of NavCam images acquired at phase angles ranging from ∼1° to 55° in order to study the nucleus reflectance properties. From empirical model fits to the phase angle dependence we obtained values of 0.065 ± 0.003 for the geometric albedo and 0.019 ± 0.001 for the Bond albedo in the broad spectral sensitivity band of the camera.