Preprints
https://doi.org/10.5194/amt-2021-88
https://doi.org/10.5194/amt-2021-88

  01 Apr 2021

01 Apr 2021

Review status: this preprint is currently under review for the journal AMT.

Accuracy in starphotometry

Liviu Ivănescu1, Konstantin Baibakov1,2, Norman T. O'Neill1, Jean-Pierre Blanchet3, and Karl-Heinz Schulz4 Liviu Ivănescu et al.
  • 1Centre d’applications et de recherches en télédétection (CARTEL), Université de Sherbrooke, Sherbrooke, QC, Canada
  • 2Canadian Space Agency | Agence spatiale canadienne, Saint-Hubert, QC, Canada
  • 3Department of Earth and Atmospheric Sciences, Université du Québec à Montréal (UQÀM), Montréal, QC, Canada
  • 4Dr. Schulz & Partner GmbH, Falkenberger Str. 36, Buckow, Germany (end of operations as of April 2016)

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.

Liviu Ivănescu et al.

Status: open (until 27 May 2021)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse

Liviu Ivănescu et al.

Liviu Ivănescu et al.

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Short summary
Starphotometry seeks to provide accurate measures of nocturnal optical depth (OD). It is driven by a need to characterize aerosols & their radiative forcing effects during a very sparse data period. A sub 0.01 OD error is required to adequately characterize key aerosol parameters. We found approaches for sufficiently mitigating errors to achieve the 0.01 standard. This renders starphotometry the equal of daytime techniques & opens the door to exploiting its distinct star-pointing advantages.