Accuracy in starphotometry

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.

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 are the low uncertainty of the star photometer measurements (0.01 AOD with the two-star method [3]) and the same clear sky conditions for simultaneous data point comparisons. This would allow an absolute calibration and the uncertainty of the exposed method could be better determined.…”

“…Finally, star photometers allow the measurement of the AOD with the use of bright stars as the light source, either with the one star method (OSM), which uses the same methodology as sun photometers, or the two stars method (TSM), which evaluates the relative brightness difference between both stars at different elevations. However, multiple challenges make it difficult to obtain stable and reliable results with this method [3].…”

Multi-star calibration in starphotometry

“…Calibration strategies for retrieving accurate photometric observations in variable optical depth conditions were proposed by Rufener (1964Rufener ( , 1986. Those studies were recently updated and complemented using measure-ments from our High Arctic, sea-level observatory at Eureka, NU, Canada (Ivȃnescu, 2015;Baibakov et al, 2015;Ivȃnescu et al, 2021), using a commercial-spectrometer-based starphotometer 1 , attached to a Celestron C11 telescope. This, more recent, work underscored certain challenges in performing calibration at such a high-latitude/low-altitude site.…”

“…Star-dependent (one-star) Langley calibration that depends on large air mass variations is the current standard in starphotometry (see Pérez-Ramírez et al, 2008. This is mainly due to the limited accuracy of available extraterrestrial star magnitudes (Ivȃnescu et al, 2021). A good number of High Arctic stars cannot, however, be calibrated in such a way since they do not go through large elevation (i.e.…”

“…air mass) changes (in the extreme case of a site at the pole, there are no elevation changes). Our goal is to demonstrate that a sub-0.01 optical depth error (partly linked to calibration errors) can be achieved by performing the type of instrument-dependent, star-independent calibration referred to in Ivȃnescu et al (2021).…”