A defect of a lens which causes the image of a point to appear as a line, rather than a point


Related Terms

ASTIGMATIZER

A lens which introduces astigmatism into an optical system. Such a lens is so arranged that it can be placed in or removed from the optical path at will. In a sextant, an astigmatizer may be used to elongate the image of a celestial body into a horizontal line.

M SYNCHRONIZATION

A linking arrangement between a camera lens and the flashbulb unit to allow a 15-millisecond delay of the shutter so that the bulb burns to its brightest point before the shutter opens.

INVERTING TELESCOPE

An instrument with the optics so arranged that the light rays entering the objective of the lens meet at the crosshairs and appear inverted when viewed through the eyepiece without altering the orientation of the image. See also ERECTING TELE- SCOPE.

LENS

A piece of glass or transparent material with plane, convex, or concave surfaces adapted for changing the direction of light rays to enlarge or reduce the apparent size of objects.

ASTIGMATIZER

A lens which introduces astigmatism into an optical system. Such a lens is so arranged that it can be placed in or removed from the optical path at will. In a sextant, an astigmatizer may be used to elongate the image of a celestial body into a horizontal line.

EYEPIECE

In an optical device, the lens group which is nearest the eye and with which the image formed by the preceding elements is viewed.

FIELD LENS

A lens at or near the plane of a real image, to collect and redirect the rays into another part of the optical system; particularly, the eyepiece lens nearest the object, to direct the rays into the eye lens.

FOCAL PLANE

A plane parallel to the plane of a lens or mirror and passing through the focus.

FOCUS

1. The point at which parallel rays of light meet after being refracted by a lens or reflected by a mirror. Also called FOCAL PO-I NT. 2. A point having specific significance relative to a geometrical figure. See under ELLIPSE, HYPERBOLA, PARABOLA. 3. The true center of an earthquake, within which the strain energy is first converted to elastic wave energy.

OBJECTIVE

The lens or combination of lenses which receives light rays from an object, and refracts them to form an image in the focal plane of the eyepiece of an optical instrument, such as a telescope. Also called OBJECT GLASS.

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