Improved energy resolution for VHE gamma ray astronomy with systems of Cherenkov telescopes

We present analysis techniques to improve the energy resolution of stereoscopic systems of imaging atmospheric Cherenkov telescopes, using the HEGRA telescope system as an example. The techniques include (i) the determinalion of the height of the shower maximum, which is then taken into account in the energy determinalion. and (ii) the determination of the location of the shower core with the additional constraint that the direction of the gamma rays is known a priori. This constraint can be applied for gamma ray point sources, and results in a significant improvement in the localization of the shower core. which translates into better energy resolution. Combining both techniques, the HEGRA telescopes reach an energy resolution between 9% and 12%. over the entire energy range from I TeV to almost 100 TcV. Options for further improvements of the energy resolution are discussed.