Celestron CR-150 HD instruction manual Polar Alignment, Equator, Definition, Astronomy Basics

Polar Alignment

Definition:

In order for the telescope to track the stars it must meet two criteria. First, you need a drive motor that will move at the same rate as the stars. For the CR-150 HD there are two optional motor drives (#93518 and #93523) that can be fitted to it. The second thing you need is to set the telescope’s axis of rotation so that it tracks in the right direction. Since the motion of the stars across the sky is caused by the Earth’s rotation about its axis, the telescope’s axis must be made parallel to the Earth’s axis.

Polar alignment is the process by which the telescope’s axis of rotation is aligned (made parallel) with the Earth’s axis of rotation. Once aligned, a telescope with a clock drive will track the stars as they move across the sky. The result is that objects observed through the telescope will appear stationary (i.e., they will not drift out of the field of view). If your telescope does not use a motor drive, all objects in the sky (day or night) will drift out of the field. This apparent motion is caused by the Earth’s rotation. Even if you are not using a motor drive, polar alignment is still desirable since it will reduce the number of corrections needed to follow an object and will limit all corrections to one axis (R.A.). There are several methods of polar alignment, all of which work on a similar principle, but are performed somewhat differently. Each method will be considered separately, beginning with the easier methods and working to the more difficult, but more precise.

Although there are several methods mentioned here, you will never use all of them during one particular observing session. Instead, you may use only one if it is a casual observing session. Or, if you plan on astrophotography, you may use two methods — one for rough alignment followed by a more accurate method.

The polar axis is the axis around which the telescope rotates when moving the telescope in right ascension. This axis remains stationary as the telescope moves in right ascension and declination.

	Zenith			axis
	the		polar		celestial
			the
	of
	Direction		of
	Direction
	North	Pole
Latitude	Horizon
							of
	North		Direction
						celestial
	Latitude		the				Pole
				North

Equator

South

Earth

Figure 5-3

When the telescope’s axis of rotation is parallel to the Earth’s axis, stars viewed through the telescope appear stationary when using a motor drive.

30 • Astronomy Basics