The optical calibration of the MAGIC telescope camera

Abstract

A 17-m-diameter air Cherenkov telescope, dubbed MAGIC, for ground-based gamma-ray astronomy above 30 GeV is under construction on the Canary Island of La Palma. The 577-pixel photomultiplier camera requires precise and regular calibration over a large dynamic range. A system for the optical calibration consisting of a number of very fast and powerful light emitting diode light pulsers is presented. We intend to calibrate each individual pixel up to 2000-3000 photoelectrons with different wavelengths, e.g. 370, 460, and 520 nm. We aim to achieve an absolute calibration at these three wavelengths by comparing the signal of the pixels with the one of specially prepared and calibrated photomultipliers and, at a later stage, hybrid photon detectors operated in single photon counting mode and with the use of well-calibrated attenuation filters (blind pixels). The light flux of the pulser is cross calibrated by a 1 cm/sup 2/ PIN diode (Hamamatsu), read out via a charge sensitive preamplifier. The PIN diode is calibrated with 60-keV gammas (from an Am 241 source) producing a precise signal of 16 570 electron-hole pairs. In addition, there will be a computer adjustable continuous light source to simulate and calibrate the response of the PMTs in the camera to the moon and the light of the night sky. This will also help to handle starlight in the field of view of the camera during analysis. Special attention is paid toward building a robust field usable device complying with IP 65 standards (sealed against heavy jet spray and dust).