Development of a new type of metallic mirrors for 21m MACE γ-ray telescope

Abstract

Major atmospheric Cherenkov experiment (MACE) is a ground-based imaging atmospheric Cherenkov telescope installed at Hanle (~4.3 km above sea level), Ladakh in the northern region of India. With a large parabolic reflector of 21 m diameter and 25 m focal length, MACE telescope is expected to explore the γ-ray Universe above 20 GeV. The tesselated light collector of MACE telescope employs 356 mirror panels each of size ~1m × 1m. The individual panel comprises of 4 metallic mirror facets each of size ~0.5m × 0.5m with a similar focal length. All 1424 mirror facets (356 × 4) are aligned in such a way that the whole reflector functions approximately as a single quasi-parabolic mirror of area ~339 m² with a focal length varying from 25 m in the central region to 26.16 m on the periphery. Here, we describe the methodology for developing the metallic mirrors using the diamond turning technique. We also present the results from the testing and characterization of ~1500 mirror facets, which qualify all the optical requirements of the MACE reflector. The testing of mirrors includes dimensionality, water ingress and environmental tests. The optical characterization of individual mirror facets is based on the measurements of focal length, spot size and reflectance.