Inauguration: 29 September 1935
Aperture: 40 cm, Focal lenght: 600 cm
Image scale: 34"/mm
In the early 1930s, plans for a new Specola at Castel Gandolfo were prepared by the firm Carl Zeiss of Jena. Construction began in 1932 and by 1935 it was mostly complete. The new visual refractor was installed under a large wooden dome 8.5 meters in diameter resting on the massive round construction of the palace’s ancient spiral staircase. Zeiss provided a 40 cm objective of 6 m focal length, together with a set of 9 eyepieces and various accessories. The instrument was also provided with a Graff photometer for observing variable stars and with a micrometer for measuring double stars. Later on a Danjon stellar interferometer was added. This was constructed in the Specola workshop and could be used for the determination of distances and positions of double stars and the diameters of planets.