  Imaging Air Cherenkov Telescopes (IACTs) detect the Cherenkov light flashes
of Extended Air Showers (EAS) triggered by very high energy (VHE) gamma-rays
impinging on the Earth's atmosphere. Due to the overwhelming background from
hadron induced EAS, the discrimination of the rare gamma-like events is rather
difficult, in particular at energies below 100 GeV. The influence of the
Geomagnetic Field (GF) on the EAS development can further complicate this
discrimination and, in addition, also systematically affect the gamma
efficiency and energy resolution of an IACT. Here we present the results from
dedicated Monte Carlo (MC) simulations for the MAGIC telescope site.
Additionally we show that measurements of sub-TeV gamma-rays from the Crab
nebula are affected even for a low GF strength of less than 33 micro Tesla.

Astrophysics