Trigger and Data Acquisition Systems

In this section I outline how VERITAS converts the light signal from the PMTs into a digital signal and records it for analysis. Even though observing is done during dark, moonless nights, there will always be a considerable level of background light entering the PMTs from nearby man-made sources and stars. VERITAS uses a three-level triggering system to suppress the recording of images of background light.

The signal from the PMT travels through a coaxial cable to a trailer that houses telescope electronics, where the signal is split three ways, each containing an identical copy of the signal from the PMT. One of the copies of the signal is amplified by a factor of 6 through the high gain circuit, digitized by an flash analog-to-digital converter (FADC), and streamed into a

CFD x 499 (1 per pixel)

Pattern Trigger Shower Delay ( 1 PDM channel per trigger signal)

Array Trigger Coincidence Logic (SAT Board)

FIFO Buffer

Compensating Delay ( 1 PDM channel per trigger signal)

Event Decision

Event Information

Harvester Process

Serialized Event Information

L3 Trigger

FADC Modules (1 channel per pixel)

(x4 : 1 per telescope)

Readout instructions

Assembled telescope data

L3 Trigger signal 64µs circular buffer Telescope Data Acquisition System L1 L2 L3 BUSY level Event Logic Inhibitor

Figure 4.7: Schematic diagram of the VERITAS trigger system (Weinstein, 2007). 16,000 sample circular RAM buffer. The FADC integrates the voltage in each 2 ns sample window, converting the integrated signal into digital counts (dc), a unit proportional to the number of photoelectrons entering the PMT in that time window. The nominal conversional gain is 0.128 dc/mV for this circuit. The FADCs, however, are only able to measure analog signals of negative polarity. To make all readings artificially negative, an offset of nominal value 117 mV is subtracted from the signal prior to digitization. The sample rate of the VERITAS FADCs is 500 megasamples/s (Buckley et al., 2003), so the buffer holds 32µ sec of data at a time.

A second identical copy of the signal is attenuated and sent through a time delay circuit. This is referred to as the low gain channel. The attenuated signal gets written to the buffer if the original signal exceeds & 120 photoelectrons and saturates the FADC. The nominal

conversional gain for this circuit is 0.128 dc/mV.

The third copy of the signal is sent to the first level of the trigger system, or the pixel level (L1) trigger. The L1 trigger for each pixel is composed of a constant fraction discriminator (CFD), a block diagram of which is pictured in Figure 4.7. An individual pixel will only trigger if its PMT pulse exceeds a certain threshold voltage, which can be adjusted depending on conditions of the night sky. The CFDs employ a constant fraction threshold rather than a fixed threshold to determine the trigger time to reduce the jitter in that timing resulting from different pulse amplitudes. An additional circuit called the rate feedback (RFB) is incorporated into the CFD to stabilize the trigger rate under changing noise levels. It achieves this by increasing (decreasing) the trigger threshold under increasing

Figure 4.8: An example FADC trace of an air shower observed by VERITAS. Plotted is the integral of all pixel charges in a single telescope sampled in 2 ns windows. The fast rise time can be seen at the fourth sample. The telescope level (L3) trigger signal is sent back to the data acquisition (DAQ) system, which reads from the FADCs and the data are written to harvester.

(decreasing) noise levels. A schematic diagram of the CFD including the RFB circuit is shown in Figure 4.9.

Figure 4.9: Schematic diagram of the CFD, including the RFB circuit (Hall et al., 2003). The camera level (L2) trigger system recognizes when three or more adjacent pixels occur within a time coincidence window, which gives preference to compact Cherenkov showers radiated by gamma rays and prevents the recording of isolated pixel fluctuations. The typical rise time of Cherenkov pulses is 3–5 ns, so the width of this time coincidence window is typically 5–6 ns.

The final level of trigger is the L3 trigger, which occurs when two or more telescopes trigger within a certain coincidence time window. The actual arrival time of the L2 signals must be corrected for the differing cable lengths transmitting the signals. There is also a correction to account for the different times that the Cherenkov light front reaches each telescope; this correction depends on the telescope pointing direction. Despite these corrections, there is still a residual spread in arrival times of the L2 signal depending on multiple factors. These factors include the width and curvature of the developing Cherenkov wavefront, the L2 timing jitter for different image sizes, and variations in timing of various electronic components. To allow for this spread, the L3 system coincidence time window is currently set to a nominal value of 50 ns. The L3 trigger is then sent back to the telescopes through modules that delay

the logic signal so that it is received a fixed time relative to the L2 trigger in each telescope. Upon receiving this signal, the telescope initiates the data acquisition (DAQ) system.

The DAQ system reads data from the FADC’s ring buffer, looking back 1500 samples corresponding to approximately 3µs, depending on the telescope. The ring buffer cannot be read and written to simultaneously, so after every event trigger there is a period of dead-time during which no events can be triggered. This dead-time reduces the effective time of observation by roughly 10%, and is subtracted from the exposure in the analysis. The L3 trigger also tags array-level information such as the event number, a GPS timestamp, and other information on a FIFO (first in, first out) basis. A software process called the Harvester collects the L2 and L3 information, along with the FADC data, and compiles it into a single event. These events are also displayed by live analysis software called Quicklook used to monitor the run in real time. Once the run has completed, the events are compiled into a raw data file, and information about the run is stored in a mySQL database. The data file is formatted in the VERITAS bank format (VBF), which is a binary data file format designed by our group. The raw data includes the digitized signal traces in the PMTs recorded by the FADCs, tracking of the telescope pointing direction, and GPS time stamps for each event, as well as a header containing the run information.