The optical system of the H.E.S.S. imaging atmospheric Cherenkov telescopes. Part II: mirror alignment and point spread function

Mirror facets of the high energy stereoscopic system imaging atmospheric Cherenkov telescopes are aligned using stars imaged onto the closed lid of the photomultiplier camera, viewed by a CCD camera. The alignment procedure works reliably and includes the automatic analysis of CCD images and control of the facet alignment actuators. On-axis, 80% of the reflected light is contained in a circle of less than 1 mrad diameter. The spot widens with increasing angle to the telescope axis. In accordance with simulations, the spot size has roughly doubled at an angle of 1.4° from the axis. The expected variation of spot size with elevation due to deformations of the support structure is visible, but is completely non-critical over the usual working range. Overall, the optical quality of the telescope exceeds the specifications.