Using stars to determine the absolute pointing of the fluorescence detector telescopes of the Pierre Auger Observatory

We describe in this paper two methods to accurately determine the absolute pointing of the fluorescence detector (FD) telescopes of the Pierre Auger Observatory. Our aim is to provide reliable and precise procedures to check and monitor the absolute telescope’s pointing and its long-term stability during the whole life of the project, estimated to be about 20 years. The two methods described, called “single pixel method” and “star track method”, are based on the FD capability to be sensitive to signals left by stars traversing the telescope field of view. As the star positions in the sky are known with very high precision, the identification of the star signals provides a powerful tool to check the telescope alignment. We used data acquired during regular observation periods in 2004 and we demonstrated that the two methods presented in this paper provide compatible sets of pointing directions.