  Usually the Imaging Atmospheric Cherenkov Telescopes, used for the
ground-based gamma-ray astronomy in the very high energy range 50 GeV - 50 TeV,
perform air shower observations till the zenith angle of ~60 deg. Beyond that
limit the column density of air increases rapidly and the Cherenkov light
absorption starts playing a major role. Absence of a proper calibration method
of light transmission restrained researchers performing regular measurements
under zenith angles >>60 deg. We extend the observation of air showers in
Cherenkov light till almost the horizon. We use an aperture photometry
technique for calibrating the Cherenkov light transmission in atmosphere during
observations under very large zenith angles. Along with longer in time
observations of a given source, this observation technique allows one to
strongly increase the collection area and the event statistics of Cherenkov
telescopes for the very high energy part of the spectrum. Study of the spectra
of the highest energy gamma rays from a handful of candidate sources can
provide a clue for the origin of the galactic cosmic rays. We show that MAGIC
very large zenith angle observations yield a collection area in excess of a
square kilometer. For selected sources this is becoming comparable with the
target collection area anticipated with the Cherenkov Telescope Array.
