the general name for optical devices used for visual observation of distant objects. Spyglasses, telescopes, binoculars, periscopes, rangefinders, sights, and geodetic instruments are included in this category. Optic tubes have been known since the turn of the 17th century. A 32-power optic tube was constructed and first used for astronomical research by Galileo in 1609. An optic tube differing from the Galilean type was proposed in 1610–11 by J. Kepler (it was first constructed about 1630).
The main elements of optic tubes are the objective and the eyepiece. The objective of an optic tube is a collecting system (usually consisting of two cemented lenses, less frequently a multilens or catadioptric system). It gives a real reduced and inverted image of a distant object near its own focal plane. The image is viewed through the eyepiece as if through a magnifying glass by matching it with the focal plane of the eyepiece. In the most widely used optic tubes (the Keplerian type; see Figure l,a), the eyepiece is also a collecting system and the image produced is inverted. Astronomical and geodetic optic tubes in which the orientation of the image is not critical are constructed according to this pattern. If an erect image must be produced, an optical erecting system (a prism system in binoculars, or a lens system in old spyglasses, periscopes and, in general, systems whose length can be great) is placed between the objective and the ocular of a Keplerian optic tube. The plane of the real intermediate image created by the objective in a Keplerian tube lies between the objective and the ocular, and a measuring scale (for example, crosshairs) or a photographic plate may be placed in it. Therefore, only this type of optic tube is used for observations that require exact measurements. The eyepieces of modern Keplerian optic tubes generally have a wide field of view (up to 90°-100°); they must be corrected for astigmatism, field curvature, coma, and chromatic aberration. Therefore, such eyepieces are usually complex systems consisting of two or more lenses. Galilean optic tubes (Figure l,b) give an erect image. A diverging lens placed in front of the plane of the intermediate real image serves as its eyepiece. Such optic tubes have a small angle of vision and are now used infrequently, mainly in opera glasses. The angular optical magnification of optic tubes for ground observations is no more than several dozen, but it may be as high as 500 or more in large telescopes. The field-of-vision angle is the largest in optic tubes that have an erection system.
REFERENCESTudorovskii, A.I . Teoriia opticheskikh priborov, 2nd ed., parts 1–2. Moscow-Leningrad, 1948–52.
Landsberg, G. S. Optika, 4th ed. Moscow, 1957. (Obshchii kurs fiziki, vol. 3.)
G. G. SLIUSAREV