The 1.2m Cassegrain telescope of the Astronomical Station Kryoneri, made by Grubb Parsons Co., was installed in 1975. Its original optical system consisted of a parabolic primary mirror of 1.2m diameter and f/3 focal ratio, and a hyperbolic secondary mirror (31 cm). The mirrors were made of Zerodur material (a type of glass with zero expansion). Its mounting is based on a modified torque-tube equatorial system. During its first 40 years of operation the telescope’s effective focal ratio was f/13, with a field of view about 40’ and an image scale of 12.5″/mm.
In May 2016, the telescope was upgraded by DFM Engineering Inc. for the needs of the NELIOTA project. This upgrade included the conversion of the original optical configuration to a prime focus telescope, the replacement of some mechanical parts of the telescope, the installation of a new control system, dome automation, and the installation of a Prime Focus Instrument (PFI) able to provide a large field-of-view as required for NELIOTA. The new optical configuration yields a focal ratio of f/2.8 (Xilouris et al. 2018).