Abstract
Abstract. Starphotometry, the night-time 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 catalogue of extraterrestrial references is noted as a major source of errors with an attendant recommendation that its accuracy, particularly its spectral photometric variability, be significantly improved. The small field of view (FOV) employed in starphotometry ensures that it, 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 arcsec (arcseconds) was found to be the best trade-off 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 made with starphotometers similar to our High Arctic instrument should be made at high angular elevations (i.e. at air masses less 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 of 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.
Highlights
The nocturnal monitoring of semi-transparent atmospheric features, such as particles or gases (O3, H2O), can be performed using attenuated starlight in order to derive a spectral optical depth (OD)
We should note that the standard starphotometry integration times (6 s) are similar to those employed for Fig. 2 short exposure times: the reason that we create the long-duration spot size is to adequately characterize the lowfrequency component of the turbulence
For a perfectly aligned star, that the maximum seeing that the field of view (FOV) can accommodate is 16.7 arcsec, for our C11 Arctic telescope
Summary
The nocturnal monitoring of semi-transparent atmospheric features, such as particles (aerosols, optically thin clouds) or gases (O3, H2O), can be performed using attenuated starlight in order to derive a spectral optical depth (OD). The accuracy of the optical depth (OD) retrieval remains critical for (second spectral order) particle feature extraction methods, which require sub-0.01 optical depth precision error, as shown in Fig. 4 of O’Neill et al (2001). Strategies for retrieving accurate photometric observations under variable optical depth conditions were proposed by Rufener (1964, 1986) and Gutierrez-Moreno and Stock (1966)1 Those fundamental astronomical studies remained largely unreferenced in atmospheric science literature. This paper consists of instrumental descriptions and a comprehensive development of OD retrieval methods followed by a detailed discussion of the error sources associated with each key OD retrieval parameter. It concludes with recommendations for achieving the 0.01 OD error goal. We avoid the non-linear complications associated with measurements in the water vapour absorption bands (in the neighbourhood of 940 nm): this subject has already been extensively described in the studies of Galkin and Arkharov (1981), Halthore et al (1997), and Galkin et al (2011, 2010)
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