Okayama Astrophysical Observatory
The Catalogue of Dry Plates with Newton Focus of the188-cm Reflector
1960〜1986
岡山天体物理観測所
188cm反射望遠鏡ニュートン焦点撮像乾板カタログ
1960〜1986

About

Okayama Astrophysical Observatory is located on Mt. Chikurinji, less than 400 meters above sea level, about 10 km inland from the coast. The Chugoku Mountains lie to the north of the observatory, the Shikoku Mountains to the south, and the Seto Inland Sea stretches between them.

This geographical condition makes the climate of the Okayama Astrophysical Observatory stable and less susceptible to sudden weather changes. The clear sky rate at Okayama Astrophysical Observatory is high, reaching 50% of observations [1,2]. The typical value of natural seeing is 1.1-1.2 arcseconds, 20% of which are sub-arcseconds [2].

Latitude +34° 34' 37.47" N
Longitude 133° 35' 38.24" E
Altitude 372 m

This is a 188 cm (or 74 inch) telescope purchased from Grubb Parsons of England in 1960. It is still one of the largest optical infrared telescopes in Japan, and at the time of its opening was the seventh largest in the world and the largest in the East. It took about five years to build and cost about 300 million yen at the time, including the building. Sister telescopes built by Grubb Parsons were also installed in Canada, Australia, France, and Egypt, with the exception of the Australian telescope, which is still in existence. The primary mirror has an aperture ratio of f/4.9, and the optical tube has a long octagonal truss structure. The telescope mount is an English cross axis equatorial, and the current pointing accuracy is as high as 10 arcseconds. The telescope has three foci, and the instruments and secondary mirror are interchanged according to the purpose of observation. The size of the moon's image at each focus is 9 cm in diameter at the Newtonian focus, 34 cm at the Cassegrain focus, and 54 cm at the Coude focus.

Telescope type: Classical Cassegrain(Grabb Parsons Inc., UK)
Weight: 50 t
Mirror Surface shape: Paraboloid
Focal length: 9.15 m F/4.9
Mirror Material: Pylex
Mirror Thickness: 27 cm
Mirror Weight: 1.7 t
Haltmann constant: 0.23 arcsec
Mounting: English cross-axis equatorial
Pointing accuracy : 10 arcsec

Two different sizes of photographic plates were used for direct imaging at Newtonian focus. One is 82 mm × 119 mm, covering a field of view of 31 arcmin × 45 arcmin. The other is 160 mm × 160 mm with a field of view of 1° × 1° square. These plates are referred to by the series names NS and NL, respectively.

Since the primary mirror of the 188cm telescope has a parabolic shape, coma aberration increases with distance from the center of the field of view, resulting in a larger star image diameter. Therefore, the NS series was used for observations near the center of the field of view, where the effect of coma aberration can be ignored. The main targets of the NS series were star clusters, nebulae, and extra galaxies. The NL series, on the other hand, was used for observations that take advantage of a wide field of view by using a coma corrector [3]. The corrector averages out the enlargement of the star image over the entire field of view, preventing the degradation of photometric and positional measurement accuracy. The NL series was used to search for unidentified objects and for astrometry of moving objects such as comets and asteroids. The successful optical identification of the brightest X-ray star Sco X-1 in 1966 is one of the most notable achievements of the NL series.

References:

• [1] Okaya Astrophysical Observatory 40th Anniversary Publication (2001), Edited by the Okaya Astrophysical Observatory 40th Anniversary Publication Committee (in Japanese)
• [2] Okaya Astrophysical Observatory 50th Anniversary Publication (2010), Edited by the Okaya Astrophysical Observatory 50th Anniversary Publication Committee (in Japanese)
• [3] Shibasaki, H., Oshima, N., & Ohtsuka, K. (1977) On the field correction of the 188cm reflector telescope, Tokyo Astronomical Observatory Report, Vol. 18, No. 1, pp. 76-85.5 (in Japanese)

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