Concept
Setting circles are used on telescopes equipped with an equatorial mount to find celestial objects by their equatorial coordinates, often used in star charts and ephemerides. Setting circles consist of two graduated disks attached to the axes – right ascension (RA) and declination (DEC) – of an equatorial mount. The RA disk is graduated into hours, minutes, and seconds. The DEC disk is graduated into degrees, arcminutes, and arcseconds. Since the RA coordinates are fixed onto 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 using setting circles is similar to finding a location on a terrestrial map using latitude and longitude. Sometimes the RA setting circle has two scales on it: one for the Northern Hemisphere and one for the Southern. 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: Polar alignment – The telescope must be aligned with either the north celestial pole or the south celestial pole. Polaris is roughly at the north pole, while Sigma Octantis is roughly at the south pole. Setting Right Ascension – After polar alignment, the observer uses a calculator or a known star to synchronize the right ascension circle with Sidereal Time. Accuracy of pointing the telescope can be hard to achieve.
