Detection of Cherenkov Light from Low Energy γ-Showers by Extra Large Diameter IACT

The time spread of the rays collected by a large diameter imaging atmospheric Cherenkov telescope (d∼20-30 m) is investigated. We compare the influence of the geometry of the spherical and parabolic reflectors on the shape and duration of collected Cherenkov light. It is shown that the design-induced additional widening of the collected Cherenkov light pulse is essential for the spherical reflector of Davies-Cotton type, and is negligibly small for the parabolic reflector. At the detection of 10 GeV γ-showers, the pulse width on the spherical telescope's focal plane may reach 15-20 ns instead of 5-8 ns inherent to the incident light pulse itself. In the case with a parabolic telescope the pulse widths of the light incident on the telescope disk and collected onto the camera are almost identical