| 摘要 |
These thick elements are all corrected for third order spherical aberration and they are also corrected for spherochromatism (i.e. chromatic variation of spherical aberration) or longitudinal color or both. Most are preferably made of materials of low index of refraction less than 1.55, such as BK-7 or fused silica. Most elements corrected for third order spherical and spherochromatism exhibit longitudinal color, but this poses no problem when they are used with a single wavelength laser. Such systems can be used to focus a laser beam to provide a very fast f number diffraction-limited point source, or used in optical testing, experiments, and the like. The designs are characterized by an extremely low sensitivity of performance to wavelength changes and thus to uniform temperature changes (thermal soaks). A temperature change of several hundred degrees centigrade-from cryogenic temperatures up to near the melting point of the optics, has essentially no effect on the predicted performance. Since the systems have no aspheric surfaces, they are substantially less expensive than fast f number parabolic mirrors, which would also provide point sources. The systems are also less alignment sensitive than parabolas of the same f number. Several systems have low fifth order spherical aberration which can be fully corrected if divided into two elements. Other systems useful in interferometric testing with lasers are described. Some employ multiple passes through a single optical element. These may be corrected for third spherical aberration, longitudinal color, and spherochromatism. Such an element with an index of 1.71 is described.
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