The Absolute Calibration of the HiRes Detectors

The HiRes experiment studies ultra high energy cosmic rays using the air fluorescence technique. The experiment uses large mirrors that collect the fluorescence light and focus it onto arrays of photomultiplier tubes (PMTs). The PMTs measure the intensity and time of arrival of the collected light. Our primary system for in situ calibration of the PMTs uses a high stability ( 16,000 PMTs). To determine the absolute response it is necessary to understand the absolute light output of this source. We have measured the source irradiance using a hybrid photodiode system, two NIST calibrated photo-diodes, and by observing the photoelectron statistics of the PMTs.