In recent years VHE gamma-ray astronomy has seen significant advances in technique ([Ong 1998];[Catanese & Weekes 1999]). There are nine groups now using the Cherenkov imaging technique (e.g., CAT, [Barrau et al. 1998]; CANGAROO, [Hara et al. 1993]; HEGRA, [Konopelko et al. 1999]; 7TA, [Aiso et al. 1997]). These techniques are relatively mature and the results from observations, with different telescopes, of the same source at the same time are consistent ([Protheroe et al. 1997]). Vigorous observing programs are now in place at all of these facilities. An important milestone for the field has been reached in that both galactic and extragalactic sources have been reliably detected ([Weekes 1999]).
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To fully exploit the potential of ground-based gamma-ray astronomy the detection techniques must be improved. This will happen by extending the energy coverage of the technique and by increasing its flux sensitivity. Ideally one would like to do both but in practice there must be trade-offs. Reduced energy threshold can be achieved by the use of larger, but cruder, mirrors and this approach is currently being exploited using existing arrays of solar heliostats (STACEE ([Chantell et al. 1998]) and CELESTE ([Giebels et al. 1998])). A German-Spanish project (MAGIC) ([Barrio et al. 1998]) to build a 17m aperture telescope has also been proposed. These projects may achieve thresholds as low as 20-30GeV where they will effectively fill the current gap in the gamma-ray spectrum from 20 to 200GeV but with poor energy resolution and small fields of view. Ultimately this gap will be covered by GLAST with weaker point source sensitivity at the higher energies. This next generation gamma-ray space telescope is scheduled for launch in 2005 by an international collaboration ([Gehrels & Michelson 1999]). Extension to higher energies (>10TeV) can be achieved by atmospheric Cherenkov telescopes working at large zenith angles and by particle arrays at very high altitudes. The MILAGRO water Cherenkov detector in New Mexico ([Sinnis et al. 1995]) operates 24 hours a day with a large field of view and has good sensitivity to gamma-ray bursts and transients.
The primary objective of VERITAS will be to have high sensitivity in
the 100GeV to 10TeV range. The German-French HESS (initially four
and eventually perhaps sixteen 12m class telescopes) will be built
in Namibia ([Konopelko 1999]) and the Japanese CANGAROO-IV array
(with four telescopes in Australia) ([Matsubara 1997]) will have
similar objectives for observations in the southern hemisphere.
MAGIC, HESS and CANGAROO are approved projects with target completion
dates in 2001 to 2004. The arrays will exploit the high sensitivity of
the atmospheric Cherenkov imaging technique and the high selectivity
of the array approach. The relative flux sensitivities as a function
of energy are shown in Figure 19, where the
sensitivities of the wide field detectors are for one year and the ACT
are for 50 hours; in all cases a 5
point source detection is
required. The VERITAS sensitivity is taken from §4
(cf., Figure 18).
It is apparent from this figure that, on the low energy side, VERITAS will complement the GLAST mission and will overlap with STACEE and CELESTE. At its highest energy it will overlap with the Tibet Air Shower Array ([Amenomori et al. 1997]). It will cover the same energy range as MILAGRO but with greater flux sensitivity. As a northern hemisphere telescope VERITAS will complement the coverage of neutrino sources to be discovered by AMANDA and ICE CUBE at the South Pole.