micrometer

In general, any instrument for measuring small distances accurately. In astronomy, micrometers attached to telescopes are used by visual observers to measure the angular separations of pairs of objects, such as binary stars. In the filar micrometer, the distance between two fine wires or markers viewed in an eyepiece can be adjusted and measured by means of a screw mechanism. In the double-image micrometer, the adjustment required to a split lens or birefringent crystal to bring images into coincidence or arrange them in a particular pattern is calibrated in terms of the angular separation of the pair of objects.