National Astronomical Observatory of Japan

Solar Tower Telescope Coelostat


Solar Tower Telescope Coelostat

The Solar Tower Telescope (nicknamed the Einstein Tower) was commissioned in 1928. It is one of the foundation telescopes prepared around the time that the Tokyo Astronomical Observatory relocated from Azabu to Mitaka in September of 1924. The coelostat in this picture was manufactured by Carl Zeiss Optical Works in Germany. This telescope played a pivotal role in spectroscopic observations of Sun until the mid-1960s, leading Japanese astrophysics. After this role was completed, it lay dormant for a long time. But thanks to the Archive Office, dome restoration and drive system repair were conducted. This is the appearance of the equipment after it was renovated in 2013.

The Evolution of the Solar Tower Telescope

The Solar Tower Telescope uses the coelostat installed in the fifth story rooftop dome to steer light into the tower itself which acts as the body of the telescope. At first, the coelostat had two German 60-cm-aperture glass plane mirrors made by Carl Zeiss Optical Works; but from 1953 to 1957 it was renovated with fused silica plane mirrors having a low thermal expansion coefficient produced by Nippon Kougaku*. Additionally, the refractive telescope was replaced with a 48-cm-diameter chromatic-aberration-free Cassegrain reflector telescope with an effective focal length of 22 m, also produced by Nippon Kougaku.

The major scientific achievements include studying the profile of a magnesium triplet spectral line, measuring the rotation of the Sun through the Doppler Effect, studying the Zeeman Effect in sunspots, and research on the atomic spectra of sunspots.

Restoration proceeded in the hands of the Archive Office which was established in 2008. Dome restoration was conducted in 2012; telescope drive system repair was carried out; an image of the Sun was formed at the focus; and now a spectrum formed by the prism spectrograph can be seen. The Solar Tower Telescope building was designated a registered tangible cultural property of Japan in 1998.

Solar Spectrum seen at the Solar Tower Telescope

The Solar Tower Telescope has had its dome and drive system repaired, and is now able to successfully form an image of the Sun on the spectrograph slit. It can observe the Sun’s spectrum via the spectrograph equipped with three large prisms. On Mitaka Open House Day, October 23 & 24, 2015, please come and see the Solar Tower Telescope which has been restored to how it was almost 50 years ago.

Text by : Masao Nakagiri
Translation by: Ramsey Lundock (NAOJ)

(* Translator’s Note: Literally “Japan Optics.” This name was used until 1948, when the company changed its name to Nikon Corporation.)