Aerosol Measurements with the FRAM Telescope

¹ Institute of Physics of the Czech Academy of Sciences, Prague
² Astronomical Institute of the Czech Academy of Sciences, Ondřejov
³ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, Warsaw
⁴ Faculty of Mathematics and Physics, Charles University, Prague

^* e-mail: ebr@fzu.cz
^** Full author list: http://www.auger.org/archive/authors_2016_09.html

Published online: 17 May 2017

Abstract

Precision stellar photometry using a telescope equipped with a CCD camera is an obvious way to measure the total aerosol content of the atmosphere as the apparent brightness of every star is affected by scattering. Achieving high precision in the vertical aerosol optical depth (at the level of 0.01) presents a series of interesting challenges. Using 3.5 years of data taken by the FRAM instrument at the Pierre Auger Observatory, we have developed a set of methods and tools to overcome most of these challenges. We use a wide-field camera and measure stars over a large span in airmass to eliminate the need for absolute calibration of the instrument. The main issues for data processing include camera calibration, source identification in curved field, catalog deficiencies, automated aperture photometry in rich fields with lens distortion and corrections for star color. In the next step, we model the airmass-dependence of the extinction and subtract the Rayleigh component of scattering, using laboratory measurements of spectral sensitivity of the device. In this contribution, we focus on the caveats and solutions found during the development of the methods, as well as several issues yet to be solved. Finally, future outlooks, such as the possibility for precision measurements of wavelength dependence of the extinction are discussed.