How to Use a Telescope With Setting Circles

By eHow Hobbies, Games & Toys Editor

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Setting circles on a telescope mount aid in locating objects in the sky. There will be two circles. One, the right ascension axis (RA for short), corresponds to the polar axis. Right ascension axes are typically measured in hours and minutes, but can also be measured in degrees. The other circle is the declination axis (DEC for short). Declination is the number of degrees off the celestial equator that an object appears. The Pole Star is near declination 90.

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Instructions

Difficulty: Moderate

Things You’ll Need:

Star Charts
Telescope Eyepieces
Telescope Tripods
Telescopes

Step1

Use setting circles when you know the coordinates of the object. Coordinates can be obtained from star charts and tables of objects in the sky.

Step2

Make sure your telescope is polar aligned before you use the setting circles. (See "How to Polar Align a Telescope" in the Related eHows.)

Step3

Locate a known star that is not too far from the object being sought. The star should be easy to spot and listed in a catalog or star chart.

Step4

Look up the coordinates of the star (or determine them from a star chart). The more accurate the coordinates, the better.

Step5

Move the setting circles to match the values obtained for the star.

Step6

Check the declination setting circle. It should require no correction. If it does, you are not polar aligned or your declination axis has slipped around its holder. Now is a good time to fix it.

Step7

Correct the RA axis. It will most likely require correction unless you are using a permanent observatory or have previously set up for another object.

Step8

Rotate the disk(s) so that the correct values for the object match on your telescope.

Step9

Look up the coordinates of the deep sky object you want to observe.

Step10

Move the telescope so that the coordinates of the telescope match those of the object. If you have done a very precise job of aligning the telescope and setting circles, the object should be in or very near your field of view.

Tips & Warnings

Use a low-power eyepiece when first trying to locate the object, and expect the object to be dim.
If the object is not in the field of view, then look around the area you are pointing at to see if you can spot it. (See "How to Locate a Deep Sky Object.") A good star chart is most helpful.
It's a good idea to use a star near the object to be located. If your alignment is not quite perfect, you will still be close.
Setting circles on most telescopes are not as precise as most charts.