Improving the H.E.S.S. angular resolution using the Disp method

Abstract

The angular resolution of imaging atmospheric Cherenkov telescopes depends on the employed event reconstruction methods. By taking the weighted average of intersections of shower axes, the H.E.S.S. experiment achieves a 0.08 degree angular resolution at 20 degree zenith angle with an image size cut of 160 p.e. for sources with a spectral index of 2. However, the angular resolution degrades to 0.14 degree at 60 degree zenith angle, due to the larger fraction of nearly parallel images. The Disp method reduces the impact of parallel images by including an estimation of the image displacement (disp), inferred from the Hillas parameters, in the reconstruction procedure. By using this technique, the angular resolution at large zenith angles can be improved by 50%. An additional cut on the estimated direction uncertainty can further improve the angular resolution to around 0.05 degrees at the expense of a loss of 50% of effective area. The performance of this reconstruction method on simulated gamma-ray events and real data is presented.