Abstract
Accurate, NIST-traceable standard stars and a calibrated Moon are essential to upward looking ground- and space-based astronomical observations, to downward looking observations of Earth, and everything in between. We assert that ground-based calibration and maintenance of standardized celestial objects is technically valid, is cost-effective, has historically been the origin of standardized celestial objects, and will continue to contribute standardized objects into the foreseeable future. Maintenance of the standard star catalog includes strategically scheduled repeated spectroradiometric observations that ensure repeatability and thus provable, random observational errors, as well as improving the accuracy of the calibrations. Earth’s atmosphere, the obvious major source of systematic error in the calibration process, is a wavelength-, direction-, and time-dependent turbid medium through which all ground-based telescopes observe. Transmission through the atmosphere is a significant source of systematic radiometric error for all terrestrial calibration campaigns which can best be obviated by direct, real-time measurements of the column of atmosphere through which a telescope is observing. Using lidar, weather, and imaging radiometric data we describe and demonstrate the effects that the atmosphere has on spectroradiometric observations. We assess the wavelength, angular and time scales on which these absorption and scattering effects operate and estimate the resulting loss of radiometric precision. We further demonstrate application of a suite of facility-class small instruments that supports calibration observations by monitoring in real-time the column of atmosphere through which the telescope is observing. This instrument suite includes a lidar, the spectroradiometer and optical/infrared cameras. We assert that application of atmospheric metadata provided by this instrument suite corrects for a significant fraction of systematic errors currently limiting radiometric precision, and provides a major step towards measurements that are provably dominated by random noise, ultimately at the fundamental photon shot noise limit.
How Earth’s Atmosphere Affects Ground-based Celestial Calibrations, and How to Correct for It
Accurate, NIST-traceable standard stars and a calibrated Moon are essential to upward looking ground- and space-based astronomical observations, to downward looking observations of Earth, and everything in between. We assert that ground-based calibration and maintenance of standardized celestial objects is technically valid, is cost-effective, has historically been the origin of standardized celestial objects, and will continue to contribute standardized objects into the foreseeable future. Maintenance of the standard star catalog includes strategically scheduled repeated spectroradiometric observations that ensure repeatability and thus provable, random observational errors, as well as improving the accuracy of the calibrations. Earth’s atmosphere, the obvious major source of systematic error in the calibration process, is a wavelength-, direction-, and time-dependent turbid medium through which all ground-based telescopes observe. Transmission through the atmosphere is a significant source of systematic radiometric error for all terrestrial calibration campaigns which can best be obviated by direct, real-time measurements of the column of atmosphere through which a telescope is observing. Using lidar, weather, and imaging radiometric data we describe and demonstrate the effects that the atmosphere has on spectroradiometric observations. We assess the wavelength, angular and time scales on which these absorption and scattering effects operate and estimate the resulting loss of radiometric precision. We further demonstrate application of a suite of facility-class small instruments that supports calibration observations by monitoring in real-time the column of atmosphere through which the telescope is observing. This instrument suite includes a lidar, the spectroradiometer and optical/infrared cameras. We assert that application of atmospheric metadata provided by this instrument suite corrects for a significant fraction of systematic errors currently limiting radiometric precision, and provides a major step towards measurements that are provably dominated by random noise, ultimately at the fundamental photon shot noise limit.