  Cherenkov Telescope Array (CTA) is an upcoming instrument that will start a
new generation of atmospheric Cherenkov telescopes. CTA is expected not only to
provide an unprecedented sensitivity in the tens of GeV to hundreds of TeV
range, but also to considerably improve the systematic uncertainties of the
measurements. We study the images registered by Cherenkov telescopes from low
energy gamma rays with its first interaction in the upper parts of the
atmosphere. The images show a characteristic separation due to the deflection
of the first e- e+ pair in the Geomagnetic Field. We evaluate the performance
of the standard stereoscopic analysis for such events. We derive also a novel
method for energy estimation of V-shaped events based purely on geometrical
properties of the image. We investigate the potential of combining the
classical energy estimation and the novel method for independent validation of
the systematic shifts in the energy scale of Cherenkov telescopes and discuss
the limitations of such analysis.
