What are setting circles?
February 20, 2005
When you look at an equatorially mounted telescope, you will notice dials on both axes of the scope. These graduated circular scales are the setting circles and are used to measure or set the pointing angles in the right ascension (RA) and declination (Dec) coordinates.
To use setting circles:
1. Make sure the scope is polar-aligned.
2. Locate a bright star, center it in your eyepiece.
3. Clamp the scope’s axes. Dial in the star's coordinates in RA and Dec to set the circles to the sky. Now you can directly move to another object simply by loosening the clamps and turning the scope until the indicators on the two axes read the new object’s RA and Dec.
If the telescope has a clock drive, most likely the circles are also driven and the reading on the RA circle won’t change as the object moves to the west. For manual scopes, the RA circle may need to be reset to maintain accuracy.
Digital setting circles allow you to computerize your telescope. By using either tiny gears or friction wheels attached to the mount, they measure the angle the telescope turns as you move it–displaying it in digital format and using this information to know where the scope is pointing. The best models have thousands of objects in their databases and allow you to turn the scope and easily find faint galaxies and stars.