Abstract
The idea of using stellar photometry for atmospheric monitoring for optical experiments in highenergy astrophysics is seemingly straightforward, but reaching high precision of the order of 0.01 in the determination of the vertical aerosol optical depth (VAOD) has proven difficult. Wide-field photometry over a large span of altitudes allows a fast determination of VAOD independently of the absolute calibration of the system, while providing this calibration as a useful by-product. Using several years of data taken by the FRAM (F/(Ph)otometric Robotic Atmospheric Monitor) telescope at the Pierre Auger Observatory in Argentina and about a year of data taken by a similar instrument deployed at the planned future Southern site of the Cherenkov Telescope Array in Chile, we have developed methods to improve the precision of this measurement technique towards and possibly beyond the 0.01 mark. Detailed laboratory measurements of the response of the whole system to both the spectrum and intensity of incoming light have proven indispensable in this analysis as the usual assumption of linearity of the CCD detectors is not valid anymore for the conditions of the observations.
Highlights
In the previous AtmoHEAD conference, we have provided a detailed description [1] of the method to measure the vertical aerosol optical depth (VAOD) by comparing the apparent brightness minst of a large amount of stars observed on a series of images from a wide-field telescope with the expected value mcat extracted from a catalog
The overall atmospheric extinction is extracted from a fit of the data with a model that includes both atmospheric and instrumental effects and can be written, for each star, as: minst = Mmcat + Zi + kiA + c1(B − V)(c2(B − V) + 1)+
Where B − V is the color index of the star, r its distance from the center of the chip, M, c1, c2, R1, R2, kC, kA2 are global parameters that depend on the experimental setup, Zi is the calibration constant for each scan and ki is the extinction coefficient from which the VAOD can be deduced after subtracting the effects of the molecular atmosphere
Summary
In the previous AtmoHEAD conference, we have provided a detailed description [1] of the method to measure the vertical aerosol optical depth (VAOD) by comparing the apparent brightness minst (in magnitudes) of a large amount of stars observed on a series of images from a wide-field telescope with the expected value mcat extracted from a catalog. These images are usually taken to cover a wide range of airmasses A in a vertical or slanted “scan”. Note that all of the measurements presented here are taken in the photometric B filter at an effective wavelength for VAOD determination between 430 and 440 nm depending on the type of aerosols and the specific hardware setup
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