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Figure 18. The R.A. and Dec. axes of the Atlas Pro
AZ/EQ-G mount.
N.C.P.
Big Dipper
(in Ursa Major)
Figure 19. Polaris is easy to find in the northern sky by extending
an imaginary line from the two "pointer stars" of the Big Dipper.
Polaris lies within 1 degree of the north celestial pole (NCP).
Eyepiece
Alignment
focus ring
setscrew (3)
Figure 20. The polar axis finder scope, which comes preinstalled
in the mount.
Polar axis
scope
Little Dipper
(in Ursa Minor)
Polaris
Cassiopeia
Focus lock
Objective
ring
lens
4. Polar Alignment
4.1
What Is It?
When you look at the night sky, you no doubt have noticed that
the stars appear to move slowly from east to west over time.
That motion is actually caused by the Earth's rotation (from
west to east). An equatorial mount is designed to compensate
for that motion, allowing you to easily "track" the movement of
astronomical objects, thereby keeping them from drifting out of
your telescope's field of view while you're observing or imaging.
This "tracking" is accomplished by slowly rotating the tele-
scope on its right ascension (R.A.) axis (Figure 18), using the
built in motor drive. But first the R.A. axis of the mount must
be aligned with the Earth's rotational (polar) axis—a process
called polar alignment.
For Northern Hemisphere operation, approximate polar align-
ment is achieved by pointing the mount's right ascension axis
at the North Star, or Polaris. It lies within 1° of the north celes-
tial pole (NCP), which is an extension of the Earth's rotational
axis out into space. Stars in the Northern Hemisphere appear
to revolve around the NCP.
To find Polaris in the sky, look north and locate the pattern of
the Big Dipper (Figure 19). The two stars at the end of the
"bowl" of the Big Dipper point in the general direction of Polaris.
Observers in the Southern Hemisphere aren't so fortunate to
have a bright star so near the south celestial pole (SCP). The
star Sigma Octantis lies about 1° from the SCP, but it is barely
visible with the naked eye (magnitude 5.5).
4.2
The Polar Axis Finder Scope
The Atlas Pro mount comes with a polar axis finder scope
(Figure 20) housed inside the right ascension axis of the
mount. When properly aligned and used, it makes accurate
polar alignment quick and easy to do. The polar scope includ-
ed with the Atlas Pro mount can be used for polar alignment
in the Northern or Southern Hemispheres. That is, the polar
scope's reticle graphic has reference stars that are useful for
aligning in either hemisphere. Remove the cap from the eye-
piece of the polar scope to view through it.
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