Development of a filter for operation of atmospheric Cherenkov telescopes close to the Moon

A filter has been developed and tested for the ARTEMIS experiment with solar-blind photomultiplier tubes. The filter consists of CoSO4, NiSO4, and 2,7-dimethyl-3,6-diazacyclohepta-2,6-diene perchlorate solutions in water. The filter transmission is about 65% in the wavelength range 230–270 nm. The NiCo solution allows a factor of 10 reduction in the background light from the Moon and the dye introduces another factor of 20. This reduction reduces significantly the systematics due to the position of the Moon. The new arrangement allows the center of the camera to be as close as 2° from the Moon center without any bias to a specific direction from the moon at any phase.