Functional diagram of the CHEOPS calibration bench. The blue arrows represent the optical fibres used to guide the light through the calibration bench. The SSS is equipped with a system that can send the light in the monochromator or bypass it. The light beam (orange) emitted by the FPM first hits the off-axis parabolic (OAP) mirror before being folded by a flat tip-tilt mirror. After exiting the bench, the light goes through a window into the TVC and enters the payload telescope. In the FPM, part of the source flux is picked up and sent to a feedback photometer that is used to control the stabilising system of the SSS.