filar micrometer

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fi·lar mi·cro·me·ter

an ocular micrometer with a line moved by a ruled drum such that a movement of the line of 5 mcm or less may be made in relation to fixed parallel lines.
References in periodicals archive ?
Measurements, given in Im, were taken with a filar micrometer with a measuring unit of 1.
If you envision a double-star observer as a wizened old astronomer with a large-aperture refractor and a filar micrometer, then think again.
Auzout is best known for inventing the filar micrometer, a device that allows observers to make precise telescopic measurements of the sizes of planetary disks or the separation of double stars.