A filar micrometer is a device used in astronomical telescopes for astrometry measurements. The word filar derives from Latin filum, a thread. It refers to the fine threads or wires used in the device.
Construction and use
The filar micrometer consists of a micrometer and a reticle that has two fine parallel wires or threads that can be moved by the observer using a screw mechanism. At one time, it was common to use spider silk as a thread. By placing one wire over one object of interest and moving the other to a second object, the distance between the two wires can be measured with the micrometer portion of the instrument. This yields the angular distance between the two objects. A common use of filar micrometers was measuring the distance between double stars. Filar micrometers are little used in modern astronomy, having been replaced by newer techniques. However, they are still used in teaching astronomy and by some amateur astronomers.
The precursor to the filar micrometer was the micrometer eyepiece, invented by William Gascoigne. Earlier measures of angular distances relied on inserting into the eyepiece a thin metal sheet cut in the shape of a narrow, isosceles triangle. The sheet was pushed into the eyepiece until the two adjacent edges of the metal sheet simultaneously occulted the two objects of interest. By carefully measuring the position where the objects were extinguished and knowing the focal length of the eyepiece, the angular distance could be calculated. Christian Huygens used such a device.