  This contribution aims to introduce the single photo-electron system designed
to calibrate the camera of the Medium-Sized Telescopes of the Cherenkov
Telescope Array (CTA). This system will allow us to measure accurately the gain
of the camera's photodetection chain and to constrain the systematic
uncertainties on the energy reconstruction of gamma rays detected by CTA. The
system consists of a white painted screen, a fishtail light guide, a flasher
and an XY motorization to allow movement. The flashes guided by the fishtail
mimic the Cherenkov radiation and illuminate the focal plane under the screen
homogeneously. Then, through the XY motorisation, the screen is moved across
the entire focal plane of the NectarCAM camera, which consists of 1855
photo-multiplier tubes. In this contribution, we present the calibration system
and the study on its optimum scan positions required to cover the full camera
effectively. Finally, we illustrate the results of the calibration data
analysis and discuss the performance of the system.
