  Cosmic ray electrons represent a background for gamma-ray observations with
Cherenkov telescopes, initiating air-showers which are difficult to distinguish
from photon-initiated showers. This similarity, however, and the presence of
cosmic ray electrons in every field observed, makes them potentially very
useful for calibration purposes. Here we study the precision with which the
relative energy scale and collection area/efficiency for photons can be
established using electrons for a major next generation instrument such as CTA.
We find that variations in collection efficiency on hour timescales can be
corrected to better than 1%. Furthermore, the break in the electron spectrum at
~0.9 TeV can be used to calibrate the energy scale at the 3% level on the same
timescale. For observations on the order of hours, statistical errors become
negligible below a few TeV and allow for an energy scale cross-check with
instruments such as CALET and AMS. Cosmic ray electrons therefore provide a
powerful calibration tool, either as an alternative to intensive atmospheric
monitoring and modelling efforts, or for independent verification of such
procedures.
