cm Cassegrain telescope, mounted in the same 5 m dome of the previous one. Primary focal length is 2400 mm, while focal length at Cassegrain focus is 7500 mm, with a relative aperture of f/12.5. Field of view at Cassegrain focus is 20'. Main instrument also accommodates a smaller 110 mm refractor finder with a focal length of 750 mm and a field of view of 2 degrees. Between 1969 and 1980 it was the biggest telescope on the Balkans to be eclipsed by the 2 m RCC telescope of Rozhen Observatory, which held this title until 2007. The purchase of the telescope was worth 200 000 leva, an enormous value at that time. Both instruments were manufactured by the German Carl Zeiss AG. The connections established with the manufacturer later helped purchase and equip instruments for the Bulgarian National Rozhen Observatory, which also has a twin 60 cm Cassegrain telescope. Currently the 15 cm telescope is used for visitor demonstrations only. In 1994 another catadioptric telescope was added in a separate, smaller dome - a 14-inch (36 cm) Schmidt-Cassegrain model. Instrumentation In 1973 the first Bulgarian single channel electro-photometer was constructed and mounted on the main instrument, operating in photon counting regime, using the UBV photometric system of filters and five diaphragms (0.5–5 mm). It was upgraded a couple of years later with an EMI-9789 QA photomultiplier. In 1997 the photometer's preamp-discriminator module was changed with a new one built in Ukraine. It is still used occasionally for bright stars. A ST-8 CCD camera was used between 1997 and 2008. Since 2008 the main 60 cm telescope is equipped with a CCD camera FLI PL-9000, manufactured by Finger Lakes Instrumentation (USA). It uses a KAF-09000 chip with a resolution of 3056 x 3056 px without binning, and a 16-bit ADC. The pixel size is 12 µm which gives, with this telescope, a scale of 0.330 arcsec/px without binning and 1.0 arcsec/px with binning 3 x 3. The field of view is 17' x 17'. BVRcIc Johnson-Cousins standard filters are used. Observations At first the observatory has been used for satellite astrometry (1960s-1970s). Between 1964 and 1974 more than 1000 Soviet satellites were observed and data sent to Mission Control Centre in Moscow for orbit corrections. Later, during the 1970s and 1980s, Photo-electric surveys of more than 200 catalogue double and multiple galaxies (1970s-1980s) were carried out. Holmberg effect was