Use of star tracks to determine photocathode anisotropy of PMTs and absolute pointing of the Pierre Auger fluorescence detector telescopes

The fluorescence detector (FD) of the Southern Pierre Auger Observatory consists of four groups of six telescopes, concentrated in four different buildings at the periphery of the 3000 km^2 observatory site. Each telescope consists of a 3.5x3.5 m^2 mirror of 3.4-m radius, an aperture of 2.2 m diameter covered by a UV filter, a corrector ring in its periphery, and an array of PMTs, disposed as a matrix of 20 columns by 22 rows in its focal surface. The pointing accuracy should be verified regularly during the whole life of the experiment, estimated to be about 20 years. Recording background-light tracks left by stars gives important information for this scope. In addition, the response along known tracks of many PMTs can be mapped by averaging the signals left by the same star in different nights. During the engineering array phase of the experiment, all PMTs have been equipped with the optically-coupled current-mirror, a novel optoelectronic system developed to perform a highly sensitive measurement of the dc or slowly varying component of the anode current, despite the fact that the PMT is biased with cathode grounded. We proved that the FD telescopes are extremely sensitive, even to 5.4 visual magnitude stars that could be clearly recorded despite the fact that a UV filter was present at the telescope aperture.