Great refractors were often used for observing double stars and equipped with a Filar micrometer.
2.
Filar micrometers are little used in modern astronomy, having been replaced by digital photographic techniques where digital pixels provide a precise reference for image distance.
3.
Being very difficult to measure, only about 60 stellar parallaxes had been obtained by the end of the 19th century, mostly by use of the filar micrometer.
4.
A typical 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 micrometer screw mechanism.
5.
The first successful measurements of stellar parallax ( to determine the distance to a star ) were made by Friedrich Bessel in 1838 for the star atmospheric turbulence in measurements compared to a filar micrometer.
6.
A "'filar micrometer "'is a specialized eyepiece used in astronomical telescopes for astrometry measurements, in microscopes for specimen measurements, and in alignment and surveying telescopes for measuring angles and distances on nearby objects.
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