The ASM LINE OF SIGHT SKY ABSORBTION MONITOR

LOSSAM, Rationale

The sky absorbtion is estimated from the variability of the average flux of the bright star used for seeing measurements. This parameter is easy to compute because it does not involve airmass correction, neither zero point estimate. Moreover, it appears that, over 10mn time bins and neglecting source chromaticity effects, the relative rms flux variation is about identical to the rms of the extinction coefficient, in R magnitude per airmass. A study of the extinction coefficient shows cases of correlation with local meteorology.

The graph shows the correlation between the relative rms flux variation and the rms of the extinction coefficient for the month of June 2000 at La Silla. The extinction coefficient is determined by aperture photometry relative to an arbitrary reference corresponding for each star to the best conditions of the month. The method induces an error (false alarm) on the determination of non photometric sky less than 7% of the time with a 10% margin, and only 2% of the time with a 20% margin

Send comments to <Marc.Sarazin@eso.org>
Created: July 5, 2000