How do I use the setting circles on my Celestron German equatorial mount?
July 1, 2009
When you look at your 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.
The following instructions are generally applicable for our German equatorial mounts as of 2009. They are very similar for many other equatorial mounts as well.
To use setting circles follow these steps:
1. Make sure your finderscope is aligned with the main scope and that your mount is polar-aligned.
2. You will need to know the names of a few of the brightest stars in the sky. If you do not know the names, they can be learned by using the Celestron Sky Maps (#93722), a planetarium program or by consulting a current astronomy magazine. Find the RA and Dec coordinates in a star atlas, reference book or from the planetarium program’s database. Record the coordinates.
3. Align the RA setting circle by locating a bright star near the celestial equator. (The farther you are from the celestial pole the better your reading on the RA setting circle will be.) The alignment star you choose should be a bright one whose coordinates are known and easy to look up. Center the star in the finderscope. Look through the main telescope and see if the star is in the field. If not, find and center it.
4. Look up the coordinates of the star. Rotate the circle until the proper coordinates line up with the RA indicator. (If your mount has a Vernier scale, line up with the zero mark on it. See instructions elsewhere in the Knowledgebase on the use of Verniers.) The RA setting circle should rotate freely. If the circle does not move freely, loosen the setting circle thumbscrew (if your mount has one).
Now you are set on that star!
5. Once the setting circles are aligned, you can use them to find any object with known coordinates. Select an object to observe. Use your seasonal star chart or planetarium program to make sure the object you choose is above the horizon at the time of night you are observing. Record the RA and Dec coordinates.
Note: The RA setting circle should be aligned each time you want to use it to find an object. This holds true even if you are using a drive. However, you do not need to use a star each time. Instead, you can use the coordinates of the object you are currently observing. (The accuracy of your setting circles is directly related to the accuracy of your polar alignment.)
6. To find the next object, hold the telescope and release the Dec clamp. Move the telescope in declination until the indicator is pointing at the correct declination coordinate. Lock the declination clamp to prevent the telescope from moving.
7. For RA, repeat the process. Hold the telescope and release the RA clamp. Move the telescope in RA until the indicator points to the correct coordinate. Lock the RA clamp to prevent the telescope from slipping in RA.
(The telescope will track in RA as long as the drive is operating.)
8. Look through the finderscope to see if you have located the object and center the object in the finder. Look in the main scope and the object should be visible. Some faint objects may not be visible in the finder. When this happens, use a star chart of the area so that you can “star hop” through the field to your target.
This process can be repeated for each object throughout the night.