Data cleaning consists of bad weather rejection and exclusion of telescopes with technical problems.
Data from the telescope CT2, which was included last into the system and was still being tuned, were excluded from this analysis. The cleaned data set contains 232 hours of observations. Shower images from a telescope are accepted in the analysis if they contain at least 40 photoelectrons. The particle direction is determined using a stereoscopic reconstruction algorithm described in Hofmann et al. Therefore, the statistical error of the background estimate is much smaller than the error of the source distribution.