Among the powerful world-renowned telescopes located atop Mt. Mauna Kea on the Big Island of Hawaii is the Subaru telescope, a large optical-infrared reflecting telescope operated by the National Astronomical Observatory of Japan.
The Subaru telescope has the world's largest single primary mirror (8.2 m in diameter), and an optical system unprecedented for a large-scale reflecting telescope, which employs a Canon prime focus corrector lens to link images to a primary focus point.

The Subaru's primary focus has a shorter focal length than the Cassegrain and other foci, allowing it to capture bright images from a wide field of view. In conventionally designed large-scale reflecting telescopes, it had not been possible to install an optical system in the primary focus because the resulting prime focus collector lens system would be too large. To address this issue, Canon developed a prime focus corrector lens system approximately 70% smaller and 50% lighter than conventional systems, thus enabling its installation in the Subaru. With a 30 arcmin field of view, it offers a field angle more than five times wider than the Cassegrain focus, which is obtained by the primary and secondary mirrors combined.

The system also incorporates a new feature developed by Canon to accurately compensate for atmospheric dispersion, a phenomenon that occurs due to differences in refractive index encountered when light from distant stars enters the earth's atmosphere. The new feature employs two lenses having the same refractive index but made of materials with different wavelength-dispersion characteristics, which shift at right angles to the optical axis to compensate for atmospheric dispersion. This unique technology has reduced the weight of the prime focus corrector lens system.

Structure of the Prime Focus Collector Lens System of the Subaru Telescope