Carson Optical RP-300 Understanding Celestial Movement, Brief Overview of Celestial Coordinates

Understanding Celestial Movement:

In order to get the most enjoyment out of your telescope it is necessary to know the basics of how celestial objects move across the sky. Due to the ro- tation of the earth, celestial objects appear to move from East to West across the sky, much like the Sun. You will notice this movement as an object in your telescope field of view will slowly move across the field and out of view. Continuous adjustment is needed to keep an object in the field of view. This will be explained in more detail later.

Many people choose to “star-hop” when using a telescope, a quick and rela- tively easy way to start. This is a method of using easily identifiable stars and constellations to serve as reference points to find other objects in the sky. A more advanced and precise method of locating specific celestial objects is by using setting circles (Fig. 3-16,17) located on your equatorial mount (Fig. 2-10) to find the celestial coordinates of that particular object.

A Brief Overview of Celestial Coordinates:

Astronomers use a system of “celestial coordinates” similar to the Earth’s latitude and longitude system to help locate objects in the sky. All celestial objects are mapped on a “celestial sphere”, an imaginary sphere of arbitrary size concentric with the Earth. If you extend the Earth’s rotational axis to infinity, both North and South, the points at which this axis intersects the celestial sphere are known as the North Celestial Pole and the South Celestial Pole. If you project the Earth’s equator outward to the celestial sphere this would be known as the “Celestial Equator”.

The equivalent to latitude lines on the celestial sphere are known as lines of “Declination”, or “Dec.” for short. These lines are measured in degrees, min- utes and seconds. Declination readings north of the celestial equator carry a “+” sign, while readings south of the celestial equator carry a “-“ sign. Ob- jects located on the celestial equator have a 0°0’0” Declination. The North Celestial Pole has a +90°0’0” Declination while the South Celestial Pole has a -90°0’0” Declination. The star Polaris is located very near the North Celes- tial Pole and has a +89.2° Declination.

The equivalent to longitude lines on the celestial sphere are known as lines of “Right Ascension” or “R.A.” for short. These lines are measured in hours, minutes and seconds starting at the “zero” line of R.A. which passes through the constellation Pegasus. There are 24 primary lines of R.A. located at 15° intervals along the celestial equator. Right Ascension coordinates range from 0hr 0min 0sec to (but not including) 24hr 0min 0sec.

Every celestial object has a corresponding R.A. and Dec. coordinate. Given the proper coordinates, you can use the setting circles on your telescope mount to locate any celestial object. The coordinates can only be used if the telescope is first aligned with the North (or South) Celestial Pole.

Polar Alignment:

If the telescope is accurately aligned with the celestial pole, very little dec- lination adjustment will be necessary to track a celestial object. Most of the tracking can be done using the Right Ascension cable (Fig. 1-8).

To line up your telescope with the pole:

• Make sure the equatorial mount is locked in the “home” position, meaning that the optical tube assembly is parallel to the correspond- ing portion of the mount below it and that the declination axis is straight up and down with the counter weight (Fig. 2-13) in it’s lowest position. For reference, the equatorial mount in Fig. 2 is in the home position.

• Loosen the equatorial mount locking screw (Fig. 3-21) so that the entire telescope (with mount) rotates freely on the tripod.

• Rotate the entire telescope until the polar axis points due North. If you are not sure which direction is North, locate Polaris and point the polar axis towards it. Polaris is less than one degree away from the Celestial North Pole and is accurate enough for polar alignment.

• If needed, level the mount by adjusting the tripod legs accordingly.

• Determine the latitude of the area you are in. Use the latitude turn screws (Fig. 3-19) to tilt the telescope until the pointer indicates the correct latitude on the latitude dial (Fig. 3-18).

• Then fine tune the latitude turn screws (Fig. 3-19) until Polaris appears in the center of your telescope field of view.

• Do not move the telescope in R.A. or Dec. while polar aligning. These adjustments should remain locked.

• If you live in the Southern Hemisphere, you should follow these steps but point the polar axis due South and locate Sigma Octantis instead of Polaris.

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