TeV gamma-ray observations of the Crab and Mkn 501 during moonshine and twilight

TeV gamma-ray signals from the Crab Nebula and Mkn 501 were detected with t he HEGRA CTI imaging Cerenkov telescope during periods when the moon was sh ining and during twilight. This was accomplished by lowering the high volta ge supply of the photomultipliers in fixed steps up to 13%. No other adjust ments were made and no filters were used. Laser runs could not establish an y nonlinearity in the gain of the individual pixels, and the trigger rate w as uniform over the whole camera. The energy threshold was increased by up to a factor of two, depending on the amount of HV reduction. In a series of observations lasting 11.7 hours, a signal with a 3.4 sigma significance wa s detected from the Crab. During the 1997 multiple flare episode of Mkn 501 a 26 sigma combined excess was recorded during 134 hours of observations u nder various moonshine/twilight conditions. The results show that this tech nique can easily be adapted to increase the exposure of a source, which is important for sources showing rapid time variability such as AGNs or GRBs. Observations can be made up to similar to 20 degrees angular separation fro m the moon and until the moon is 85% illuminated (ten to eleven days before and after new moon), as well as during 20 to 40 minutes during twilight, b efore the commencement of astronomical darkness. (C) 1999 Elsevier Science B.V. All rights reserved.