The electronic camera for each VERITAS telescope is similar to previous imaging atmospheric Cherenkov detectors ([Cawley et al. 1990]) but with some significant improvements. An outline of the components of the camera is shown in Figure 13. The focal plane detector consists of an array of 499 separate PMTs. The signals are first amplified near the detectors in the focal plane and then transmitted to the local electronics located near the telescope base. These systems provide the local trigger and convert the analog signals to digital form. To take full advantage of the fast Cherenkov pulse structure, the split signals from the individual PMTs are fed to fast constant fraction discriminators (CFDs) to provide the signals necessary to form the initial trigger by detecting coincidences on a time-scale <15ns. The outputs from these discriminators are fed to a local pattern trigger unit which tests for adjacent detector hits in the telescope. The pattern trigger signal is sent to the central trigger location where programmable delays are applied to the trigger signal from each telescope to correct for the difference in the time of arrival of the Cherenkov wavefront at the individual telescopes due to the source location in the sky. An array trigger can be formed from a combination of individual telescope triggers at the central trigger. The trigger is designed to record all gamma-ray events that contain enough light to be usefully identified as gamma-ray events (reconstruction threshold) while limiting the trigger rate to a manageable level.
The other branch of the analog signal from each detector is fed to a flash analog to digital converter (FADC) which digitizes the detector waveform into a circulating memory. This results in a digitized version of the signal pulse from each detector. When a readout trigger decision has been made for an individual telescope, a sub-array of telescopes or the full array, the associated electronics loads the appropriate section of the FADC memory of those signal channels with hits to a data bus which is read out by a controller and stored in reflective memory modules. This information is read out by local CPUs and merged into event data for the telescope. The individual telescope data is transmitted the central station, merged with data from the other telescopes and stored. A centrally generated optical pulse is used to synchronize individual electronic channels and the time tagging of events across the array.
Some of the individual elements of the VERITAS electronic cameras are discussed briefly here.
