Large zenith-angle observations with the CAT Cherenkov imaging telescope

We present here results from large zenith-angle observations with the CAT atmospheric Cherenkov imaging telescope, based on data taken on the Crab Nebula and on the blazar Mk501 from 1996 onwards. From Monte Carlo simulations, the threshold energy of the telescope is expected to vary from about 250 GeV at zenith to about 2 TeV at a zenith angle of 60 degrees. The lower source-fluxes due to the increased threshold energy are partly compensated for by an increase in the effective collection area at large zenith angles, thus allowing a significant extension of the dynamic range of the CAT telescope, with a tolerable loss in sensitivity. We discuss the implications for source detection and energy spectrum measurements.