  The prototype of a Davies-Cotton small size telescope (SST-1M) has been
designed and developed by a consortium of Polish and Swiss institutions and
proposed for the Cherenkov Telescope Array (CTA) observatory. The main purpose
of the optical system is to focus the Cherenkov light emitted by extensive air
showers in the atmosphere onto the focal plane detectors. The main component of
the system is a dish consisting of 18 hexagonal mirrors with a total effective
collection area of 6.47 m2 (including the shadowing and estimated mirror
reflectivity). Such a solution was chosen taking into account the analysis of
the Cherenkov light propagation and based on optical simulations. The proper
curvature and stability of the dish is ensured by the mirror alignment system
and the isostatic interface to the telescope structure. Here we present the
design of the optical subsystem together with the performance measurements of
its components.
