Effects of the moonlight on the operating parameters of the MACE γ-ray telescope: a feasibility study

Major atmospheric Cherenkov experiment (MACE) is a very high-energy γ -ray telescope based on imaging atmospheric Cherenkov technique. The telescope has recently been installed at Hanle, Ladakh and results from its first light are expected very soon. The imaging atmospheric Cherenkov telescopes like MACE are usually operated during dark, clear and moonless nights and hence their duty cycle is limited to ∼ 10% (1000 hours per year). Increasing the duty cycle of these telescopes is a challenging task in the field of ground-based very high-energy γ -ray astronomy. In this work, we study the feasibility of operating the MACE telescope under moderate moonlight to monitor the variable γ -ray sources in the sky as long as possible. We have developed a detailed methodology for estimating the contribution of moonlight as a function of its phase to the night sky background at the MACE site using the model of the brightness of moonlight proposed by Krisciunas & Shaefer (1991). The effect of additional background due to moonlight is examined on the various operational parameters such as anode current of the photomultiplier tubes in the camera, discriminator threshold, threshold energy and sensitivity of the MACE telescope. For the safe operation of the photomultiplier tubes under partial moonlight conditions, we estimate the gain in observation time or duty cycle of the telescope. It is found that if the strict requirements of the imaging atmospheric Cherenkov telescopes are relaxed by operating the photomultiplier tubes in the MACE camera at a lower gain or higher anode current and hence at a relatively higher energy threshold under moderate moonlight, a significant increase of ∼ 20% in the effective observation time is expected every year for observations with the MACE telescope.