The primary mirror has an aperture of 8.2 m.
A telescop's light-gathering capability is a function of the area of its primary mirror. The primary mirror of the Subaru Telescope is one of the largest monolithic mirrors in the world, with an effective aperture of 8.2 m. Made of ultra-low-heat-expansion (ULE) glass, the mirror is 20 cm thick and weighs in at 22.8 t.
The faintest stars visible to the human eye have an apparent magnitude of 6 (log scale; higher numbers are fainter). With light-gathering capability of over a million times that of the human eye, the Subaru Telescope can observe stars with apparent magnitudes as high as 27. The glass for the primary mirror was manufactured in New York State, polished in Pennsylvania and installed in Hawaii. Painstakingly crafted over a period of 7 years, the mirror was transported to its final destination with the utmost care.