The Zeiss Double Astrograph

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 Petzval objective of 240 cm focal length and a reflecting telescope Newton-Cassegrain system 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.

Vatican Observatory 2025.09.27