Effectiveness of TeV gamma-ray observations at large zenith angles with a stereoscopic system of imaging atmospheric Cerenkov telescopes

The sensitivity of imaging atmospheric Cerenkov telescopes (IACTs) in TeV g amma-ray observations reaches its maximum at small zenith angles (SZAs) (Th eta less than or equal to 30 degrees) which provides the minimum attainable energy threshold of an instrument. However. for a specific telescope site a number of gamma-ray sources, or source candidates, can only be observed a t much larger zenith angles (LZAs) (O less than or equal to 60 degrees). Mo reover, the observations at LZAs allow us to extend the observation time wi ndow for any object seen at SZAs, as well as to enlarge the dynamic energy range of an instrument towards the highest observable energies of gamma-ray s. Based on Monte Carlo simulations we present the results on the sensitivi ty of a stereoscopic system of five IACTs in observations at LZAs. We point out some important parameters of the telescope design which could substant ially improve the efficiency of such observations with forthcoming IACT arr ays like CANGAROO III, HESS and VERITAS.