Calibration of the telescope array experiment fluorescence detectors

Abstract

The Telescope Array (TA) experiment has three stations of Fluorescence Detectors (FDs). Two stations (Black Rock Mesa BR and Long Ridge LR) consist of 24 new detectors (12 each). The third station (Middle Drum MD) consists of 14 detectors that were reconditioned and redeployed from the High Resolution Fly's Eys experiment (HiRes). In this paper, we describe the detector calibration process for the BR and LR FD stations. The telescope calibration includes the measurement of (1) mirror reflectances, (2) absolute gains of reference phototubes, (3) relative gains of the clusters of 256 tubes, (4) (non-)uniformity of photo-cathode sensitivity of the tubes, and (5) transmittance of the UV filters on the photo-tubes and acrylic windows on the cameras. The reflectances of mirrors, transmittance of the UV filters and the acrylic windows are measured using a spectrophotometer. The absolute gains and nonuniformity of photo-cathode are measured with a simple pulsed UV light calibration systems and a large XY stage with UV LEDs. The relative gains are monitored hourly during observations with a Xe flasher and an alpha-ray scintillator pulsar called YAP. We discuss the qualities of the calibration data, and we evaluate systematic uncertainties in air shower reconstruction and energy determinations.