When it came online in 1999, Japan's 26.9ft Subaru Telescope was the most expensive observatory ever constructed. The 22-ton mirror is one of the largest optical mirrors in existence. The telescope helped create a 3D map of 3000 galaxies in 2016 that shows Einstein's theory of relativity still holds true. Observatory tours (which, sadly, don't include looking through the telescope) are given in Japanese or English but not both; they fill up fast so register online early.
Children under 16 years old are not allowed on tours and there are no public restrooms.
Incidentally, Subaru is the Japanese word for the Pleiades (Seven Sisters) constellation.