National Astronomical Observatory of Japan

20-cm Telescope Dome

The 20-cm Telescope Dome was designed by Facilities Department of Tokyo Imperial University and constructed by Mr. Chodayu Nishiura in 1921. The construction of it is flat building made with reinforced concrete. The 20-cm Refractor Telescope was made by Carl Zeiss in Germany. The mount is equatorial clock drive by weight hanging system (governor style). This telescope had worked for observations through sunspot sketches for 61 years since 1938.

The 20-cm Telescope Dome is the oldest observational building in the NAOJ Mitaka campus, but the structure keeps the functionality.

This building has been registered a tangible cultural property of Japan in February, 2002.

Solar Observation

In addition, at the 20-cm Telescope Dome, solar observation is held. On sunny days, the staff controls this 20-cm Refractor Telescope, and visitors can observe the sunspots on the surface of the sun.

Open-day schedule in August, 2019
August 3, 4, 10, 11, 17, 18, 24, 25 and 31
Open-day schedule in September, 2019
September 1, 7, 8, 14, 15, 21, 22, 28 and 29
Opening hours
(July, August and September) From 10:00 a.m. to Noon.
It might be cancelled on intense heat days.
(Other Months) From 10:00 a.m. to 2:30 p.m.
How to Join
The advanced registration is not required. Everyone can join it free. The staff waits near the telescope, please feel free to ask.


20-cm Refractor Telescope
Aperture20 cm
Focal length359 cm
MountWeight clock drive equatorial
Target of observationSunspot
SizeDiameter 6m, Height 7.8m
RemarksRegistered Tangible Cultural Property (Structures)
Brief history
(Taisho era 10)
(Showa era 2)
Installation Zeiss equatorial
(Heisei era 10)
Research observation end
(Heisei era 12)
Open to the public
February 2002
(Heisei era 14)
Registered as Tangible Cultural Property of Japan