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.

April 6, 7, 13, 14, 20, 21, 27 and 28
May 4, 5, 11, 12, 18, 19, 25 and 26
From 10:30 a.m. to around 3:00 p.m. (while the sun is visible)
These events can be canceled due to inclement weather or other unforeseen circumstances.
How to Participate
No application and no fee are required.
The staff will usher each participant individually into the dome to keep the space from being overcrowded.


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