Locating TeV γ-ray Sources with Sub-Arcminute Precision: the Pointing Calibration of the HEGRA System of Imaging Atmospheric Cherenkov Telescopes

Stereoscopic viewing of TeV γ-ray air showers with systems of Imaging Atmospheric Cherenkov Telescopes (IACTs) allows to reconstruct the origin of individual primary particles with an accuracy of 0.1◦ or better. The shower impact point can be determined within 15 meters. To actually achieve this resolution, the pointing of the telescopes of an IACT system needs to be controlled with high precision. For the HEGRA IACT system, a procedure to calibrate telescope pointing was established, using bright stars distributed over the sky as references. On the basis of these measurements, one determines parameters of a correction function which is valid for the complete hemisphere. After correction a pointing accuracy of 0.01◦ is achieved.