An improved technique for the determination of shower geometry from single and stereo IACT images

In order to better utilize the information contained in the shower images generated by imaging Cherenkov telescopes (IACTs) equipped with cameras with small pixels, images are fitted to a parametrization of image shapes gained from Monte Carlo simulations, treating the shower direction, impact point, and energy as free parameters. Monte Carlo studies were carried out for stereoscopic shower images obtained with a system of IACTs. An improvement of order 1.5 in the angular resolution and in the determination of the core position is predicted. The fitting technique can also be applied to single-telescope images. Simulations indicate that the shower direction in space can be reconstructed event-by-event with a resolution of 0.16-0.20 degrees. Data from Crab observations with a single HEGRA telescope confirm this prediction.