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 settings 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.
Accuracy of pointing the telescope can be hard to achieve. Some sources of error are:
It is common to blame an unlevel tripod as a source of error, however when a proper polar alignment is performed, any induced error is factored out.
These sources of error add up and cause the telescope to point far from the desired object. They are also hard to control; for example, Polaris is often used as the celestial north pole for alignment purposes, but it is over half a degree away from the true pole. Also, even the finest graduations on setting circles are usually more than a degree apart, which makes them difficult to read accurately, especially in the dark. Nothing can be done if the optical tube is not perpendicular to the declination axis or if the R.A. and Dec axes are not perpendicular, because these problems are next to impossible to fix.
Despite these inaccuracies setting circles can be used to roughly get to a desired object's coordinates, where a star chart can be used to apply the necessary correction. Alternatively, it is possible to point to a bright star very close to the object, rotate the circles to match the star's coordinates, and then point to the desired object's coordinates. Setting circles are also used in a modified version of star hopping where the observer points the telescope at a known object and then moves it a set distance in RA or declination to the location of a desired object.
In contrast to a GOTO telescope mount, a mount equipped with DSC alone is sometimes called a "PUSH TO" mount.
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