Setting circles

Setting circles consist of two graduated disks attached to the axes – right ascension (RA) and declination (DEC) – of an equatorial mount.

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.

The term Right Ascension took its name from early northern hemisphere observers for whom "ascending stars" were on the east or right hand side.

Even with some inaccuracies in polar alignment or the perpendicularity of the mount, 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.

They give a highly accurate readout of where the telescope is pointed and their lit display makes them easier to read in the dark.