The optical calibration of the MAGIC telescope camera

Currently a 17 m diameter air Cherenkov telescope, dubbed MAGIC, for ground based gamma ray astronomy above 30 GeV is under construction on the Canarian 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 LED light pulsers is presented. We intend to calibrate each individual pixel up to 2000-3000 photoelectrons with different wavelengths, e.g. 370 nm, 460 nm 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 (PMTs), and in a later stage hybrid photo multipliers (HPD), using the single photon counting mode and a well calibrated attenuation filter (´blind pixels´). The light flux of the pulser is cross calibrated by a 1 cm² PIN diode (Hamamatsu), read out via a charge sensitive preamplifier. The pindiode is calibrated with 60 keV gammas (from an Am 241 source) producing a precise signal of 16570 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 star light in the field of view of the camera in the analysis. Special attention is paid at building a robust field usable device complying with IP 65 standards.