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.
Highlights
Most of the background light is removed by the aperture photometry method, but the dark-current correction plays an important role when we take into account that the sensitivity of the lens-camera system is not uniform across the field of view, because of the different sensitivity of each pixel, the vigneting of the lens and due to dust and other contamination on various optical surfaces
We have shown that we can use the data on apparent brightness of a large number of stars distributed across a wide range of zenith angles gathered in the Shoot-theShower program to extract the integral vertical aerosol optical depth (VAOD) values to the top of the atmosphere
The model parameters can be turned into VAOD values using theoretically calculated values for Rayleigh scattering
Summary
The Pierre Auger Observatory [1] is the world’s largest detector of ultra-high energy cosmic rays (UHECR). Where Z is the zeropoint (a calibration constant of the system), A is airmass (amount of air traversed by the light relative to a star in zenith), k is the extinction coefficient (1.086 × the vertical optical depth) and f (...) is in general a correction function of some properties of the star and the optical system. Two main things should be kept in mind: firstly that this is still a work in progress and secondly that the method has specific limitations, in particular in the fact that we always measure the integral extinction across the whole atmosphere and cannot determine the height profile of the aerosols
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