Accuracy in starphotometry

Abstract

Abstract. Starphotometry, the nightime counterpart of sunphotometry, has not yet achieved the commonly sought observational error level of 1%: a spectral optical depth (OD) error level of 0.01. In order to address this issue, we investigate a large variety of systematic (absolute) uncertainty sources. The bright star catalog of extraterrestrial references is noted as a major source of errors with an attendant recommendation that its accuracy, as well as its spectral photometric variability, be significantly improved. The small Field of View (FOV) employed in starphotometry ensures that starphotometry, unlike sun- or moonphotometry, is only weakly dependent on the intrinsic and artificial OD reduction induced by scattering into the FOV by optically thin clouds. A FOV of 45 arc-seconds was found to be the best tradeoff for minimizing such forward scattering errors concurrently with flux loss through vignetting. The importance of monitoring the sky background and using interpolation techniques to avoid spikes and to compensate for measurement delay was underscored. A set of 20 channels was identified to mitigate contamination errors associated with stellar and terrestrial-atmospheric gas absorptions, as well as aurora and airglow emissions. We also note that observations for starsphotometers similar to our high-Arctic starphotometer should be made at high angular elevations, i.e. at airmasses lower than 5. We noted the significant effects of snow crystal deposition on the starphotometer optics, how pseudo OD increases associated with this type of contamination could be detected and how proactive techniques could be employed to avoid their occurrence in the first place. If all these recommendations are followed, one may aspire to achieve component errors that are well below 0.01: in the process one may attain a total 0.01 OD target error.

Remote Sensing of Aerosols at Night with the CoSQM Sky Brightness Data

“…Future work will also aim to compare AOD results obtained from star photometry to the technique described in this paper. The advantages of this approach rely on the low uncertainty of the star photometer measurements (0.01 AOD with the 2 star method [31]) and the same clear sky conditions for simultaneous data point comparison. This would allow an absolute calibration and the uncertainty of the exposed method could be better determined.…”