Zeiss Visual

Zeiss Visual Refractor Telescope

German mounting
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.

Zeiss Double

Zeiss Double Astrograph 

Reflector: Parabolic mirror:
Aperture 60 cm, Focal l. 240 cm

German mounting
Inauguration: 29 September 1935
Refractor: Four lens objective,
Aperture 40 cm
chromatic correction 390 to 460 m
Focal l. 200 cm
Image scale 1.42'/mm
Field 8o
Photographic plate 30x30 cm2

Newton System:
plane secondary mirror,
22 cm at 45
Image scale 1'.22/mm
Field 2.6o

Cassegrain System:
convex secondary mirror
15 cm
Equivalent focal l. 820 cm
Image scale 25"/mm
Field 50'

In the second rotating dome, 8 meters in diameter, also made of wood constructed on the solid foundation offered by the northeast corner of the palace, the principal instrument of the observatory was placed: the Zeiss Double Astrograph. It consisted of a refractor with a 40 cm four-lens objective of 240 cm focal length and a reflecting telescope with a 60 cm parabolic mirror of 200 cm Newtonian focal length and an equivalent 8.2 m focal length at Cassegrain focus. Both instruments plus two finders and a guide telescope were rigidly linked together and mounted on the same polar axis.
A large spectrograph could be mounted on the reflector for astrophysical research. The four-lens astrograph allowed 30x30 cm photographs with image correction to be taken; it was particularly suitable for photographic observations of variable stars and for the photographic determination of the positions of minor planets and comets. Two large (61.2 cm diameter) flint prisms of refracting angle 4 and 8 degrees respectively could be attached singly or together at the upper end of the reflector or refractor, thereby allowing spectra to be taken over large fields.