Most equatorial mounts have numbered dials on each axis. These are manual, or analog, setting circles, and they can be used to locate an object using its right ascension and declination coordinates. However, newcomers to astronomy rarely find setting circles very helpful. The dials require careful adjustments with every use and can quickly lose accuracy as the sky moves during the night.
Computerized, or digital, setting circles, are generally more useful, though more expensive. These devices are programmed with the locations of hundreds, if not thousands, of stars, nebulas, clusters, and galaxies—and even planets on some models. Add-on digital setting circles come with a pair of encoders one tor each axis of the mount. The encoders sense the motion of the mount and keep track of how far it has moved.
Digital setting circles are easy to use and provide a highly accurate method of locating objects. You begin aiming the telescope at two bright stars on either side of the sky. These two positions are all the computer needs to know to guide you to any of the objects in its memory. Call up one of those objects and a display shows which way to swing the telescope and indicates when you are on target. Look in the eyepiece and there the object is!
Digital setting circles can be added to most of today's popular telescopes. Remarkably, they do not need a polar-aligned equatorial mount, and will work , on altazimuth or Dobsonian mounts. Most models sell for the price of a beginner's telescope.