Filar micrometer
Encyclopedia
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.
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Filar micrometers are little used in modern astronomy, having been replaced by newer techniques.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.
.
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.
Astrometry
Astrometry is the branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. The information obtained by astrometric measurements provides information on the kinematics and physical origin of our Solar System and our Galaxy, the Milky...
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 micrometerMicrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
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
Spider silk
Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons for protection for their offspring...
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
Angular distance
In mathematics and all natural sciences , the angular distance between two point objects, as observed from a location different from either of these objects, is the size of the angle between the two directions originating from...
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.
Prior devices
The precursor to the filar micrometer was the micrometer eyepiece, invented by William GascoigneWilliam Gascoigne (scientist)
William Gascoigne was an English astronomer, mathematician and maker of scientific instruments from Middleton near Leeds who invented the micrometer...
.
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.