lens

Top: The formation of a real image using a convex lens. Bottom: The formation of a real image using a concave mirror. In both diagrams, f is the focal point, O is the object, and I is the image.

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In an imperfect lens L, all the rays do not pass through a focal point. The smallest circle that they pass through C is called the circle of least confusion.

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This image may be ... viewed through an eyepiece, which acts like a magnifying glass. The eye then sees an inverted magnified virtual image of the object.

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This image may be ... viewed through an eyepiece, which acts like a magnifying glass. The eye then sees a ... magnified virtual image of the object.

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A principal focus or focal point is a special focus: For a lens, or a spherical or parabolic mirror, it is a point onto which collimated light parallel to the axis is focused.

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Objectives can be a single lens or mirror, or combinations of several optical elements.

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A telescope's ability to resolve small detail is directly related to the diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the light), and its light gathering power is related to the area of the objective.

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There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as erecting telescopes.

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Most telescope designs produce an inverted image at the focal plane; these are referred to as inverting telescopes.

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This image may be recorded or viewed through an eyepiece, which acts like a magnifying glass. The eye then sees an inverted [DISPUTED] magnified virtual image of the object.

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