Setting circles

Setting circles are used on telescopes equipped with an equatorial mount to find astronomical objects in the sky by their equatorial coordinates often used in star charts or ephemerides.

Setting circles consist of two graduated disks attached to the right ascension (RA) and declination (DEC) axis of an equatorial mount. The right ascension disk is graduated into hours, minutes, and seconds. The declination disk is graduated into degrees, minutes, and seconds. Since right ascension coordinates are fixed to the celestial sphere the RA disk is usually driven by a clock mechanism in sync with sidereal time. Locating an object on the celestial sphere with setting circles is similar to finding a location on a terrestrial map using latitude and longitude. Sometimes the right ascension setting circle has two scales on it: one for the Northern and one for the Southern hemisphere.

Historically setting circles have rivaled the telescopes optics as far as difficulty in construction. Making a set of setting circles required a lot of precision crafting on a dividing engine. Setting circles usually had a large diameter and when combined with a vernier scale could point a telescope to nearly an arc minute of accuracy. In the 20th century setting circles were replaced with electronic encoders on most research telescopes.

In amateur astronomy, setting up a portable telescope equipped with setting circles requires:

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