A filar micrometer is a specialized eyepiece used in astronomical telescopes for

astrometry Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other Astronomical object, celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, th ...

measurements, in

microscopes A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...

for specimen measurements, and in alignment and

surveying Surveying or land surveying is the technique, profession, art, and science of determining the land, terrestrial Plane (mathematics), two-dimensional or Three-dimensional space#In Euclidean geometry, three-dimensional positions of Point (geom ...

telescopes for measuring angles and distances on nearby objects. "Filar" derives from the

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

("thread"). It refers to the fine threads or wires used in the device.

Construction and use

A typical filar micrometer consists of a

reticle A reticle or reticule, also known as a graticule or crosshair, is a pattern of fine lines or markings built into the eyepiece of an optical device such as a telescopic sight, spotting scope, theodolite, optical microscope or the electronic v ...

that has two fine parallel wires or threads that can be moved by the observer using a micrometer screw mechanism. The wires are placed in the focal image plane of the eyepiece so they remain sharply superimposed over the object under observation, while the micrometer motion moves the wires across the focal plane. Other designs employ a fixed reticle, against which one wire or a second reticle moves. By rotating the eyepiece assembly in the eyetube, the measurement axis can be aligned to match the orientation of the two points of observation. At one time, it was common to use

spider silk Spider silk is a protein fibre or silk spun by spiders. Spiders use silk to make webs or other structures that function as adhesive traps to catch prey, to entangle and restrain prey before biting, to transmit tactile information, or as nest ...

as a thread.Berenbaum, May R., ''Field Notes — Spin Control'', The Sciences, The New York Academy Of Sciences, September/October 1995 By placing one wire over one point of interest and moving the other to a second point, the distance between the two wires can be measured with the micrometer portion of the instrument. Given this precise distance measurement at the image plane, a trigonometric calculation with the objective focal length yields the

angular distance Angular distance or angular separation is the measure of the angle between the orientation (geometry), orientation of two straight lines, ray (geometry), rays, or vector (geometry), vectors in three-dimensional space, or the central angle subtende ...

between the two points seen in a telescope. 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. This absolute measurement is independent of the distance to the object, due to the telecentricity principle. A common use of filar micrometers in astronomical telescopes was measuring the distance between double stars. Filar micrometers are little used in modern astronomy, having been replaced by digital photographic techniques where digital

pixels In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest addressable element in a dot matrix display device. In most digital display devices, pixels are the sma ...

provide a precise reference for image distance. Filar eyepieces 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 Gascoigne Sir William Gascoigne (c. 135017 December 1419) was Chief Justice of England during the reign of King Henry IV. Life and work Gascoigne (alternatively spelled Gascoyne) was a descendant of an ancient Yorkshire family. He was born in Gawthor ...

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

isosceles triangle In geometry, an isosceles triangle () is a triangle that has two Edge (geometry), sides of equal length and two angles of equal measure. Sometimes it is specified as having ''exactly'' two sides of equal length, and sometimes as having ''at le ...

. 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 objective lens, the angular distance could be calculated.

Christiaan Huygens Christiaan Huygens, Halen, Lord of Zeelhem, ( , ; ; also spelled Huyghens; ; 14 April 1629 – 8 July 1695) was a Dutch mathematician, physicist, engineer, astronomer, and inventor who is regarded as a key figure in the Scientific Revolution ...

used such a device.

Notes

References

* George O. Abell, ''Exploration of the Universe'', fourth edition, Saunders College Publishing, 1982,

External links

Photographs of the Filar micrometer circa 1930 used at the Lick Observatory from the Lick Observatory Records Digital Archive, UC Santa Cruz Library's Digital Collection
{{Commonscat, Filar micrometers Measuring instruments Astronomical instruments Scientific instruments Surveying instruments Historical scientific instruments History of astronomy

Construction and use

Prior devices

See also

Notes

References

External links