The Very EneRgetIc Telescope Array System (VERITAS) is located at Mt Hopkins in southern Arizona. It makes observations of gamma-rays from ~100GeV-10TeV energies by collecting light from the small air showers they produce interacting in the upper atmosphere. The effective collecting area of this technique reaches ~105 m2. The array uses stereoscopic imaging of each arriving air shower to reconstruct the shower direction and discriminate against the large backgrounds of hadronic cosmic rays. Each VERITAS telescope has a diameter of ~10m. The focal plane cameras are arrays of 499 photomultipliers that can resolve the arrival time of individual photons to ~2ns using a FADC system.
More details are available at the VERITAS collaboration website.
The Chicago group is also participating in studies and development for the next generation of ground-based instruments consisting of an array with an order of magnitude more sensitivity. This effort is called AGIS in the USA, and CTA in Europe.