Polar Alignment - Celestron C4.5 Instruction Manual

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Polar Alignment
Definition:
In order for the telescope to track the stars you must meet two criteria. First,
you need a motor drive that will move at the same rate as the Earth rotates.
As an optional accessory, Celestron offers a clock drive that also works as a
drive corrector, a device that allows you to override the rate of the motors
during astrophotography. 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 it's axis, the
telescope's axis must be made parallel to the Earth's.
Polar alignment is the process by which the telescope's axis of rotation
(called the polar axis) 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 appear stationary (i.e., they will not drift out of the field of view).
If not using the clock drive, all objects in the sky (day or night) will slowly
drift out of the field. This motion is caused by the Earth's rotation. Even if
you are not using the clock drive, polar alignment is still desirable since it
will reduce the number of corrections needed to follow an object and limit all
corrections to one axis (R.A.). There are several methods of polar alignment,
all work on a similar principle, but performed somewhat differently. Each
method is considered separately, beginning with the easier methods and
working to the more difficult.
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, you may use two methods; one for
rough alignment followed by a more accurate method if you plan on doing
astrophotography.
The polar axis is the axis around which the telescope rotates when moved in
right ascension. This axis points the same direction even when the telescope
moves in right ascension and declination.
Figure 5-3
When the telescope's axis of rotation (known as the polar or R.A. axis) is
parallel to the Earth's axis, stars appear stationary when using the
optional clock drive. The process of making the telescope's axis of
rotation parallel to the Earth's is described in the section on "Polar
Alignment." Once this is done, the telescope is pointed by moving it in
R.A. and DEC.
Astronomy Basics * 27