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filar micrometer

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A device for measuring the separations of two objects as seen through a telescope, such as the components of a double star. It consists of two fixed fine wires or webs in the field of view of an eyepiece, crossing at right angles, plus a third movable web, parallel to one of the fixed webs. The position of the movable web can be measured precisely against a scale. Many filar micrometers can also be rotated against an angular scale; such a device is known as a position micrometer. Both separation and position angle can then be measured.

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