impersonal micrometer

impersonal micrometer

[im′pərs·ən·əl mī′kräm·əd·ər]
(engineering)
An instrument consisting of a vertical wire that is mounted in the focal plane of a transit circle and can be moved across the field of view to follow a star, and instrumentation to record the position of the wire as a function of time; used to reduce systematic observational errors.