The concept for the ACTL hardware emphasizes the usage of standard, off-the-shelf networking and computing elements and works to minimize the amount of hardware (e.g. electronics cards) that must be specifically developed for CTA. It also accounts for the need to develop, maintain and operate a more complex and more stable (when compared to existing Imaging Atmospheric Cherenkov Telescope or IACT arrays) software system with limited manpower at moderate cost. It prescribes the use of software frameworks, the application of widely accepted standards, tools and protocols, and follows an open-source approach.
The ACTL hardware will comprise of:
- A single-mode fiber Ethernet wide area network (several 10 Gbit/s) connecting the telescopes with the OSDC and facilitating data transfer, array-level triggering, and clock-distribution. In addition, a local WLAN will be made available close to each telescope to facilitate access with laptops and tablets for debugging and commissioning.
- Computers (so called camera servers) located in the OSDC and assigned to one telescope to receive the data after a camera trigger. The camera servers buffer the Cherenkov data while the array trigger makes its decision.
- A central computing cluster (also located in the OSDC) for execution of ACTL software, event building and filtering, and operation of the data repository. Estimates for the number of computing cores and the capacity of the data repository are about 1550 (870) and 3 PB (1.5 PB) for CTA-southern hemisphere (CTA-northern hemisphere).
- Hardware (few computers) for a SoftWare Array Trigger (SWAT), also located in the OSDC. The SWAT inspects the telescope event time-stamps and selects stereoscopic events.
- A WhiteRabbit (WR) network connecting a central GPS clock with each telescope and the SWAT. The WhiteRabbit system provides time-stamps with sub-nanosecond precision that are used to associate data and telescope events.
- WR interface cards for time-stamping and array-level triggering, respectively, that are deployed close to any hardware using their services, in particular the cameras.
The design intentionally does not prescribe hardware standards for transfer of data and control. Dedicated protocols and Ethernet are used for the transfer of the bulk data (camera data); for all other applications, the usage of one particular software standard is enforced to ensure connectivity with many different hardware devices.