National Astronomical Observatory of Japan

20-cm Telescope Dome

The Oldest Observation Building in Mitaka Campus

20-cm Telescope Dome

The 20-cm (7.9-inch) Telescope Dome was designed by the Construction and Repair Department of Tokyo Imperial University, and constructed by Mr. Chodayu Nishiura in 1921. This two-story building is made of reinforced concrete. The 20-cm Refractor Telescope, housed in the dome, was manufactured by the German company Carl Zeiss. Its equatorial mount has a weight-driven clockwork mechanism controlled by a governor. This telescope played a major role in sketching observations of sunspots for 61 years starting from 1938.

The 20-cm Telescope Dome is the oldest among the observation buildings in Mitaka Campus, but its design embodies functionality.

This building was listed as a Registered Tangible Cultural Property of Japan in February 2002.

Solar Observation Parties

Solar observation parties are regularly held at the 20-cm Telescope Dome. When conditions are right, participants can observe sunspots on the solar surface with the 20-cm Refractor Telescope operated by the staff.

To prevent the spread of the novel coronavirus, solar observation parties continue to be canceled until further notice.
Only on sunny days from 10:00 a.m. to 2:30 p.m. or from 10:00 a.m. to Noon (in July, August, and September). In the summer season, the events may be canceled due to intense heat. After the observation parties, visitors can only view the telescope (until 5:00 p.m.).
How to Join
No fees or reservations are required. Our staff will be waiting at the telescope dome, so feel free to ask and join the party.


20-cm Refractor Telescope
Aperture20 cm (7.9 in)
Focal Length359 cm (11.8 ft)
MountWeight-driven clockwork drive
PurposeSunspot observations
SizeDiameter: 6 m (19.7 ft), Height: 7.8 m (25.6 ft)
StatusRegistered Tangible Cultural Property
Brief history
(Taisho Year 10)
(Showa Year 2)
Zeiss-made equatorial telescope installed
(Heisei Year 10)
Retired from operation
(Heisei Year 12)
Opened to the public
February 2002
(Heisei Year 14)
Listed as a Registered Tangible Cultural Property