Large zenith angle observations with the high-resolution GRANITE III camera

The GRANITE III camera of the Whipple Cherenkov Telescope at the Fred Lawrence Whipple Observatory on Mount Hopkins, Arizona (2300 m a.s.l.) has the highest angular resolution of all cameras used on this telescope so far. The central region of the camera has 379 pixels with an individual angular diameter of 0.12 degrees. This makes the instrument especially suitable for observations of gamma-induced air-showers at large zenith angles since the increase in average distance to the shower maximum leads to smaller shower images in the focal plane of the telescope. We examine the performance of the telescope for observations of gamma-induced air-showers at zenith angles up to 63 degrees based on observations of Mkn 421 and using Monte Carlo Simulations. An improvement to the standard data analysis is suggested.