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filar micrometer(fÿ -lar) (position micrometer) A measuring device incorporated into the eyepiece of an instrument and used in astronomy to measure the angular separation and orientation of a visual binary star or to determine the position of an object relative to a nearby star whose coordinates are known. It consists essentially of two parallel wires that lie in the focal plane of the eyepiece and can be rotated about the optical axis of the telescope. A third wire lies perpendicular to the parallel wires. Two stars are aligned along this perpendicular wire. The position of one of the parallel wires can be controlled by turning a screw with a finely graduated micrometer head. The parallel wires are adjusted so as to coincide with the images of the two stars and the separation of the images can then be accurately measured.
an auxiliary device used with microscopes and with geodetic, astronomical, and other instruments in order to precisely measure small distances or angles. A filar micrometer consists of a housing containing a grid of fixed wires that are mounted in the focal plane of the instrument and are viewed through the eyepiece together with the image of the object being observed. A movable wire, which can be displaced with the aid of a finely threaded screw, is superimposed on the image. The screw has a graduated drum with a scale. The scale divisions are used to read the magnitude of the displacement of the movable wire with respect to the fixed wires. In astronomical instruments that are used to take measurements in various directions of the celestial sphere, the filar micrometer is mounted such that it can be rotated around the optical axis of the tube of the instrument.