department of physics & astronomy, UCLA


VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a ground-based gamma-ray observatory located at the Fred Lawrence Whipple Observatory at the base of Mt. Hopkins, 40 miles south of Tucson, AZ. The observatory consists of an array of four Davies-Cotton reflecting telescopes, each with a diameter of 12 m. Through stereoscopic imaging of particle showers occuring in the atmosphere, the array is capable of indirectly detecting gamma rays with energies of ~85 GeV to several tens of TeV.


The array works by capturing the brief pulses of Cherenkov light produced by charged-particle showers in the Earth's atmosphere. The particle showers are produced when high-energy gamma-rays or cosmic rays interact with nuclei in the atmosphere. These interactions - predominantly pair production and bremsstrahlung produce large numbers of relativistic charged particles. Showers initiated by a gamma ray are dominated by electrons and positrons, while showers intitiated by cosmic rays contain heavier particles, notably pions and muons. Charged particles in both types of showers are moving faster than the speed of light in the atmosphere, which gives rise to the Cherenkov light detected by the telescopes. This light is uv-optical part of the electromagnetic spectrum and is the same blue light that is produced in fission reactors.

VERITAS detects gamma rays from numerous non-thermal astrophysical sources including pulsar wind nebulae, supernovae remnants, active galactic nuclei, X-ray binaries and starburst galaxies. VERITAS is also used to search for gamma rays from other potential sources such as dark matter, gamma-ray bursts, primordial black hole evaporation, and other objects. 


The Cherenkov Telescope Array (CTA) is a proposed next-generation imaging atmospheric Cherenkov telescope array. It is predicted to provide a very significant improvement in sensitivity over current-generation experiments (HESS, MAGIC, VERITAS).  CTA will consist of an array of various-sized telescopes based mostly on proven technology but on a much larger scale than the current telescope arrays. Currently there is planning underway for two sites, one each in the northern and southern hemispheres, respectively.


CTA will address some of the most interesting questions about the high-energy, non-thermal universe, including

  • Determining the origin and propogation of high-energy cosmic rays.
  • Understanding processes of particle accleleration around compact objects (e.g. black holes, neutron stars, etc.)
  • Searching for answers to questions about physics beyond the standard model (dark matter, quantum gravity, etc.)

More information, including news about CTA can be found at this link


The Milagro Gamma-Ray Observatory was a large-area, ground-based experiment designed for the indirect detection of VHE photons from astrophysical sources. Con- structed at the site of, and using some infrastructure from the former Hot Dry Rock geothermal experiment, Milagro was located in the Jemez Mountains outside of Los Alamos, NM at an elevation of 2600 m above sea level (a.s.l.). The experiment oper- ated almost continuously (total duty cycle of ∼ 95%) from 2000 January until it was decommissioned in 2008 May, during which time it surveyed at GeV – TeV energies much of the northern hemisphere.

Inside Milagro


Milagro is a member of the class of ground-based, high-energy particle detectors referred to extensive air shower (EAS) arrays which operate by the principle of de- tecting the secondary particles from EASs that are generated by energetic primary particles (including gamma rays) interacting with nuclei high in the Earth’s atmosphere.