Improving the Pointing Precision of the H.E.S.S. Experiment

The H.E.S.S. experiment consists of four Imaging Atmospheric Cherenkov Telescopes (IACTs) located in Namibia. IACTs collect the Cherenkov light emitted from the cascade of secondary particles that is created when gamma-rays interact with the atmosphere. The direction of individual gamma-rays can be measured to about 0.1°, but the average source position can be determined to higher accuracy and systematic errors in telescope pointing can emerge as the main limitation. Typical pointing accuracies for IACTs are in the order of arc-minutes. In H.E.S.S., the pointing is determined from reference measurements of stars. The telescope is pointed at a star while the Cherenkov camera is covered by a lid serving as screen for the reflection. A CCD Camera monitors the location of this reflection relative to the focal plane represented by positioning LEDs. A second CCD camera acts as guiding telescope. The systematic errors of possible influences on the pointing accuracy are discussed. An alternative pointing technique using CCD information measured in parallel to gamma-ray observations is presented and compared to the standard method. The new method approximately bisects the systematical error on the pointing of the H.E.S.S. experiment to 6''. Its validity is tested on point-like gamma-ray sources of known position