Abstract
Using the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) onboard the SOlar Radiation and Climate Experiment (SORCE), we have acquired disk-integrated absolute spectral reflectance measurements of the Moon in the wavelength range 115 – 300 nm with phase angle coverage ~0 to 170 degrees (both waxing and waning phases). SOLSTICE has the ability to measure the lunar and solar irradiance using the same optics and detectors, thereby mitigating the effects of solar variability and instrument degradation. The calculation of reflectance is independent of the radiometric sensitivity, and is proportional to the ratio of lunar to solar count rates and the throughput of the two modes. These unique characteristics of SOLSTICE allow for an accurate measure of the lunar reflectance. Because SOLSTICE was never intended to observe the Moon, there are several corrections required in order to achieve the theoretical accuracy of the measurement. Effects contributing to systematic errors include vignetting from overfilled optics, wavelength shifts due to pointing drift, and a non-trivial model of the lunar point-spread function required to match the spectral resolution of the solar data to that of the lunar data. While processing improvements continue, we have begun a scientific evaluation of the dataset. Disk-integral photometric models have been fit to the data, and compared to similar measurements at visible wavelengths. One unexpected result has been a measurement of lunar polarization. The Moon is partially linearly polarized, as is the SOLSTICE instrument. By observing the Moon at a variety of angles with respect to the scattering plane, we have been able to derive the polarization of the Moon in the wavelength range 240 – 300 nm over a phase angle range of -90 (waxing) to +90 degrees (waning). We will present an overview of these observations, corrections applied to the data, a preliminary look at the scientific analysis including photometry and polarization, and implications for use of the Moon as a flight calibration target.
Disk-integrated Measurements of the Moon in the Ultraviolet
Using the SOLar-STellar Irradiance Comparison Experiment (SOLSTICE) onboard the SOlar Radiation and Climate Experiment (SORCE), we have acquired disk-integrated absolute spectral reflectance measurements of the Moon in the wavelength range 115 – 300 nm with phase angle coverage ~0 to 170 degrees (both waxing and waning phases). SOLSTICE has the ability to measure the lunar and solar irradiance using the same optics and detectors, thereby mitigating the effects of solar variability and instrument degradation. The calculation of reflectance is independent of the radiometric sensitivity, and is proportional to the ratio of lunar to solar count rates and the throughput of the two modes. These unique characteristics of SOLSTICE allow for an accurate measure of the lunar reflectance. Because SOLSTICE was never intended to observe the Moon, there are several corrections required in order to achieve the theoretical accuracy of the measurement. Effects contributing to systematic errors include vignetting from overfilled optics, wavelength shifts due to pointing drift, and a non-trivial model of the lunar point-spread function required to match the spectral resolution of the solar data to that of the lunar data. While processing improvements continue, we have begun a scientific evaluation of the dataset. Disk-integral photometric models have been fit to the data, and compared to similar measurements at visible wavelengths. One unexpected result has been a measurement of lunar polarization. The Moon is partially linearly polarized, as is the SOLSTICE instrument. By observing the Moon at a variety of angles with respect to the scattering plane, we have been able to derive the polarization of the Moon in the wavelength range 240 – 300 nm over a phase angle range of -90 (waxing) to +90 degrees (waning). We will present an overview of these observations, corrections applied to the data, a preliminary look at the scientific analysis including photometry and polarization, and implications for use of the Moon as a flight calibration target.