Filar micrometer

By rotating the eyepiece assembly in the eyetube, the measurement axis can be aligned to match the orientation of the two points of observation.

In a microscope, a similar calculation yields the spatial distance between two points on a specimen.

In an alignment telescope, the precise micrometric measurement of the eyepiece image directly indicates the real distance of a nearby observed point from the line of sight.

A common use of filar micrometers in astronomical telescopes was measuring the distance between double stars.

Earlier measures of angular distances relied on inserting into the eyepiece a thin metal sheet cut in the shape of a narrow, isosceles triangle.