A novel apparatus to measure reflected sunlight from the Moon

Abstract

We describe a new apparatus for measuring the spectral irradiance of the Moon at visible wavelengths. Our effort builds upon the United States Geological Survey’s highly successful Robotic Lunar Observatory (ROLO), which determined a precise model for the time-dependent irradiance of the Moon from six years of observations obtained with an imaging telescope equipped with a set of narrow-band filters. The ROLO Irradiance Model allows the Moon to be used as a radiometric reference for tracking changes in the absolute responsivity of near-infrared to visible satellite sensors as a function of time to better than 1 %. The goal of the present effort is to improve the absolute radiometric accuracy of the ROLO model, presently estimated at 5 % - 10 %, to better than 1 %. Our approach, which uses an integrating sphere at the focal plane of a telescope to direct light from the integrated lunar disk into a stable spectrograph, also eliminates the need to interpolate between the 32 visible and near-infrared bands measured by ROLO. The new measurements will allow weather, climate, land-surface, and defense satellites to use the Moon as an absolute calibration reference, potentially reducing the impact of disruptions in continuous long-term climate data records caused by gaps in satellitesensor coverage.