Understanding the Setting Circles
The setting circles on an equatorial mount enable you to locate celestial objects by their “celestial coordinates”. Every object resides in a specific location on the “celestial sphere”. That location is denoted by two numbers: its right ascension (R.A.) and declination (Dec.). In the same way, every location on Earth can be described by its longitude and latitude. R.A. is similar to longitude on Earth, and Dec. is similar to latitude. The R.A. and Dec. values for celestial objects can be found in any star atlas or star catalog.
The mount’s R.A. setting circle is scaled in hours, from 1 through 24, with small marks in between representing
The Dec. setting circle is scaled in degrees, with each mark rep- resenting 2.5° increments. Values of Dec. coordinates range from +90° to
So, the coordinates for the Orion Nebula listed in a star atlas will look like this:
R.A. 5h 35.4m Dec.
That’s 5 hours and 35.4 minutes in right ascension, and
Before you can use the setting circles to locate objects, the mount must be properly polar aligned, and the R.A. setting circle must be calibrated. The Dec. setting circle has been permanently calibrated at the factory, and should read 90° whenever the tele- scope optical tube is parallel with the R.A. axis.
Calibrating the Right Ascension Setting Circle
1.Identify a bright star in the sky near the celestial equator (Dec. = 0°) and look up its coordinates in a star atlas.
2.Loosen the R.A. and Dec. lock knobs on the equatorial mount, so the telescope optical tube can move freely.
3.Point the telescope at the bright star whose coordinates you know. Lock the R.A. and Dec. lock knobs. Center the star in the telescope’s field of view with the
4.Rotate the setting circle until the metal arrow indicates the R.A. coordinate listed in the star atlas for the object.
Finding Objects with the Setting Circles
1.Now that both setting circles are calibrated, look up in a star atlas the coordinates of an object you wish to view.
2.Loosen the R.A. lock knob and rotate the telescope until the R.A. value from the star atlas matches the reading on the R.A. setting circle. Remember to use the upper set of numbers on the R.A. setting circle. Retighten the lock knob.
3.Loosen the Dec. lock knob and rotate the telescope until the Dec. value from the star atlas matches the reading on the Dec. setting circle. Remember that values of the Dec. setting circle are positive when the telescope is pointing north of the celestial equator (Dec. = 0°), and negative when the telescope is pointing south of the celestial equa- tor. Retighten the lock knob.
Most setting circles are not accurate enough to put an object
The R.A. setting circle must be
Confused About Pointing the Telescope?
Beginners occasionally experience some confusion about how to point the telescope overhead or in other directions. In Figure 1a the telescope is pointed north, as it would be during polar align- ment. The counterweight shaft is oriented downward. But it will not look like that when the telescope is pointed in other directions. Let’s say you want to view an object that is directly overhead, at the zenith. How do you do it?
One thing you DO NOT do is make any adjustment to the latitude adjustment
Similarly, to point the telescope directly south, the counterweight shaft should again be horizontal. Then you simply rotate the scope on the Dec. axis until it points in the south direction.
What if you need to aim the telescope directly north, but at an object that is nearer to the horizon than Polaris? You can’t do it with the counterweight down as pictured in Figure 1a. Again, you have to rotate the scope in R.A. so the counterweight shaft is positioned horizontally. Then rotate the scope in Dec. so it points to where you want it near the horizon.
To point the telescope to the east or west, or in other direc- tions, you rotate the telescope on its R.A. and Dec. axes. Depending on the altitude of the object you want to observe, the counterweight shaft will be oriented somewhere between vertical and horizontal.
Figure 8 illustrates how the telescope will look pointed at the four cardinal
The key things to remember when pointing the telescope is that a) you only move it in R.A. and Dec., not in azimuth or latitude (altitude), and b) the counterweight and shaft will not always appear as it does in Figure 1a. In fact, it almost never will!
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