were filtered by rejecting trivial background events, such as accidental noise triggers, triggers from nearby muons or data taken during adverse conditions (low atmospheric transmission, car light flashes etc.). Light clusters, either from large angle shower particles or from the night sky light background (stars), well separated from the main image, were removed from the images. For the events included in the analysis, the mean trigger rate was required to be constant within $\approx 20\%$. In order to improve the comparability of the two data sets from summer and winter 2005 we restricted the maximum zenith angle to $\leq 34^\circ$. From the remaining events, corresponding to 23.1 h observation time, image parameters were calculated [@Hillas_parameters] such as WIDTH, LENGTH, SIZE, CONC, and M3LONG, the third moment of the light distribution along the major image axis. For the $\gamma$/hadron separation a multidimensional classification procedure based on the random forest method was employed [@Breiman; @Bock]. The separation procedure was trained using a sample of Monte Carlo (MC) generated $\gamma$-ray shower images [@Knapp; @Majumdar] on the one hand and about 1% randomly selected events from the measured wobble data representing the hadronic background on the other hand. The MC $\gamma$-ray showers were generated 