Abstract
All sky cameras are a great alternative to the study of atmospheric conditions in an astronomical site. We show how to compute the instantaneous coefficients of atmospheric extinction for various moments in the same night. The images used were obtained by the camera SASCA (SOAR AllSky Camera) located at Cerro Pachon. To estimate the extinction coefficients we developed a method to measure the brightness of several stars at different air masses for the same instant. We developed a Python program that calculates azimuth and altitude for the stars and then converts them to pixel coordinates on the CCD. Comparing the positions calculated for a group of stars with their actual positions on the images, we can determine the distortion caused by the camera. After finding out the distortions we know exactly in which pixel a star falls on any date and time. At this point we created tables of star positions and did the photometry of them all on each image using the IRAF routine PHOT. These photometry tables are subsequently converted into multiple tables with apparent magnitude versus air mass for each star at a given instant. Our results show that the extinction coefficient calculated for different atmospheric moments is within two sigma of the values of the coefficients obtained through the monitoring of a single star at different air masses, demonstrating the feasibility of our approach.
