| 摘要 |
A principal concave mirror (12) and a dioptric correction device (14) are arranged such that the mirror is slightly inclined relative to the optical axis (OO') of an associated lens. The mirror focuses an incident light beam (20,22) to a point (F) adjacent to an objective lens by reflection via a second plane mirror (26). The dioptric corrector comprises a prismatic lens which is slightly divergent in character, and which is of decreasing thickness as it extends radially away from the principal focus (FM) of the concave mirror. The concave mirror, and the two surfaces (28,30) of the prismatic lens, are all part-spherical.
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