New developments in aerosol measurements using stellar photometry

The idea of using stellar photometry for atmospheric monitoring for optical experiments in high-energy 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.


5/19
Moon effect = background effect • sort into two classes "dark" and "bright" -"upper branch" seems related to a period of higher aerosols CCD nonlinearity: the cause of background problem?
• Relation between incoming light and ADU counts not linear -manifests as non-linear measured/catalog magnitude relation • Stronger for smallest fluxes -> explains correlation with background following effect had been discovered on the frames with low mean signal levels, which looks like signiÞcant non-linearity of the detector. Basically, faint stars appear to be signiÞcantly fainter than expected from the best Þt calibration model, which includes catalogue values, color term and low-order spatial polynomial.
The amount of this effect (characterized by a Pearson correlation coefÞcient of calibration residuals vs magnitude for the range of magnitudes between V=8 and V=12) clearly depends on the mean signal level of the bias subtracted image, and, therefore, on the input of CCD preamp or ADU convertor. Also, it systematically manifests, in a bit different but similar ways, on both CCDs used on FRAM over last two years, and therefore does not look like the hardware failure.
The amount of this effect (characterized by a Pearson correlation coefÞcient of calibration residuals vs magnitude for the range of magnitudes between V=8 and V=12) clearly depends on the mean signal level of the bias subtracted image, and, therefore, on the input of CCD preamp or ADU convertor. Also, it systematically manifests, in a bit different but similar ways, on both CCDs used on FRAM over last two years, and therefore does not look like the hardware failure.

7/19
The imaging sequence consists frames with randomly chosen e distribution between 0.1 and 10^ dark frame with the same exposu acquiring every 2 pairs (twice lig +dark pair is acquired with 10 s e control on light level and bias s 1k region has been used to spee Data processed with Non-linearity Correction (NLC) How to choose the cuts?