Measuring sky brightness in the presence of the Moon

Presenter(s): James Irish

Affiliation(s): Citizen astronomer, ASV

Session details: Sunday, 2020-04-12 04:30:00, SUN

Abstract

Astronomers, unless they are lunar aficionados, don’t like moonlight or clouds. But, just as the Sun illuminates the daytime, excites solar observers and enables sun photometry, the Moon can also be used as the luminance source for measuring sky brightness. SQM measurements under moonlight reveal how much trees or other obstructions affect data in the absence of the Moon. Example plots will be presented. Data from multiple sites has been used to devise a simple model of the relationship between the zenith distance of the Moon and the SQM measurement. Departures from that model indicate changes in extinction coefficient for scattered light or obstructions such as trees. I’ll describe more sophisticated models from the literature on the brightness over a narrow field of view as a function of the arc distance to the Moon and the phase of the Moon; these enable visual and imaging astronomers to plan observations close to the Moon’s trajectory. Finally, I’ll note that the standard sun photometer (CIMEL318) is now available in a version which can be used with the Moon as the luminance source, enabling measurement of the aerosol optical depth at multiple wavelengths for about two weeks each lunar cycle.

Biography

Jim has been using Unihedron sky brightness meters for several years, at multiple sites. He gave a presentation at the previous NACAA on how light from far off-axis affects measurements. He is trialing other methods of measuring the severity of airglow. He lives in SE Queensland at a dark site close to Warwick, and shares the house with numerous telescopes.