IACT

Imaging Atmospheric Cherenkov Telescope, or IACT, refers to a class of ground-based telescopes used in very-high-energy gamma-ray astronomy. IACTs detect the faint, brief flashes of Cherenkov light produced when gamma rays with GeV to TeV energies interact with the Earth’s atmosphere and create extensive air showers. The light is collected by large segmented mirrors and focused onto fast cameras, typically composed of photomultiplier tubes or silicon photomultipliers. By imaging the shower from multiple telescopes, scientists reconstruct the gamma-ray arrival direction and energy and distinguish gamma-ray–induced showers from the far more numerous cosmic-ray–induced showers.

Development of the method began in the 1980s, with the first major gamma-ray source detections using imaging atmospheric Cherenkov techniques in the late 1980s. The approach matured into stereoscopic arrays in the 2000s, with facilities such as VERITAS in the United States, H.E.S.S. in Namibia, and MAGIC in the Canary Islands delivering substantially improved sensitivity and angular resolution. The next-generation Cherenkov Telescope Array (CTA) is under construction and will comprise numerous telescopes of different sizes across the northern and southern hemispheres to extend the accessible energy range and sky coverage.

Typical IACTs employ mirror diameters of about 10 to 24 meters, with fields of view of a few degrees. They achieve angular resolutions around a tenth of a degree and energy resolutions in the 10–20 percent range, depending on energy and configuration. Observations require dark, clear skies and are conducted on moonless nights, which limits the duty cycle but enables very sensitive measurements at high energies.

Current major facilities include H.E.S.S., VERITAS, and MAGIC. The CTA project aims to provide a substantial leap in sensitivity and spectral coverage for gamma-ray astronomy.