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Realization
of Large Aperture and Wide Field of View Thanks to Prime Focus Corrector Lens |
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| The summit of Mt. Mauna Kea on Hawaii Island,
4,200 meters above sea level, boasts 10 astronomical observatories with large
telescopes, and is globally renowned as the Mecca of astronomical observation.
Among these huge, advanced telescopes rising to the sky stands the large infrared
telescope known as Subaru, which is operated by the Japanese National Astronomical
Observatory. Subaru's primary mirror has an aperture of 8.2 meters, the largest
of a single mirror in the world, and is equipped with an optical system to form
images on the prime focus that could not be realized with conventional large reflecting
telescopes. The prime focus can obtain images with a wider field of view because
the focal length is shorter in comparison with other focuses, including the Cassegrain
focus, which forms an image with the light reflected from the primary mirror using
the secondary mirror. Previously, optical systems heavy at the upper end could
not be placed in large reflecting telescopes, because they would become a major
structural burden. For observation purpose, a system allowing smooth changes of
the prime focus optical system and secondary mirror was also required. Canon's
prime focus corrector lens cleared these problems. |
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About 70%
Smaller, and 50% Lighter than Conventional Systems |
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| Canon's prime focus corrector lens succeeded
in achieving compactness and lightweight-70% or less in size and 50% or less in
weight-compared with conventional designs. Despite these features, the lens consists
of 5-group and 7-element large lenses, and the maximum lens diameter (520 mm)
and total weight (170 kg) make this the largest lens unit produced by Canon. Using
this lens, the moon, as seen from end to end from the earth, can be caught in
a field of view of 30 minutes. In other words, the prime focus corrector lens
provides 25 times the area of the Cassegrain focus, the field of view of which
is about six minutes. For example, when the whole sky is observed, the difference
in visual field is such that, while it would take about 2,500 years to observe
the entire sky using the Cassegrain focus, only 100 years would be required in
the case of the Subaru's prime focus corrector lens. Subaru is a telescope with
a far wider field of view than its cousins. |
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Large Correction
of Atmospheric Dispersion: the Enemy of Observation |
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| Canon's prime focus corrector lens has another
great feature. It can accurately correct for atmospheric dispersion, which causes
coloring, because the refraction index differs depending on the wavelength when
light enters the atmosphere. The initial design included an atmospheric dispersion
correction system made by attaching two triangular prisms in two sets, and then
rotating them. However, this method required four thick glass sheets serving only
the purpose of correcting atmospheric dispersion. At Canon, we devised a system
to correct atmospheric dispersion by combining two lenses using materials with
the same refractive index, but different in wavelength dispersion, and then shifting
the lenses perpendicularly to the optical axis. As a result, the overall weight
of the system was greatly reduced. |
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Lens Manufacturing Know-How Made the Prime Focus Corrector Lens a Reality Canon overcame a variety of barriers to manufacture the prime focus corrector lens, not only in the design, but also in manufacturing processes. The very high processing accuracy required to produce two large-aperture aspherical lenses, and the severe restriction that the clearance between lens and titanium barrel must be dozens of mm when seven lenses are contained in the barrel, were successfully cleared by applying our high-level lens manufacturing know-how. The completed prime focus corrector lens system was installed in the Subaru, and test observations were conducted in August 1999. Full-scale operation starts in 2001. In addition, Canon has advanced into the fields of space development and astronomical observation, including satellite optical systems. In 1994, we launched the SO (Space Optics) Project to specialize in these areas of development. |
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