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Little Dipper. They point to Polaris (see Figure 6-6). The position of the Big Dipper changes during the year and
throughout the course of the night (see Figure 6-5). When the Big Dipper is low in the sky (i.e., near the horizon), it may be
difficult to locate. During these times, look for Cassiopeia (see Figure 6-6). Observers in the southern hemisphere are not as
fortunate as those in the northern hemisphere. The stars around the south celestial pole are not nearly as bright as those
around the north. The closest star that is relatively bright is Sigma Octantis. This star is just within naked eye limit
(magnitude 5.5) and lies about 59 arc minutes from the pole.
The north celestial pole is the point in the northern hemisphere around which all stars appear to rotate.
The counterpart in the southern hemisphere is referred to as the south celestial pole.
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This method of polar alignment allows you to get the most accurate alignment on the celestial pole and is required if
you want to do long exposure deep-sky astrophotography through the telescope. The declination drift method requires
that you monitor the drift of selected stars. The drift of each star tells you how far away the polar axis is pointing from
the true celestial pole and in what direction. Although declination drift is simple and straight-forward, it requires a
great deal of time and patience to complete when first attempted. The declination drift method should be done after
any one of the previously mentioned methods has been completed.
To perform the declination drift method you need to choose two bright stars. One should be near the eastern horizon
and one due south near the meridian. Both stars should be near the celestial equator (i.e., 0° declination). You will
monitor the drift of each star one at a time and in declination only. While monitoring a star on the meridian, any
misalignment in the east-west direction is revealed. While monitoring a star near the east/west horizon, any
misalignment in the north-south direction is revealed. It is helpful to have an illuminated reticle eyepiece to help you
recognize any drift. For very close alignment, a Barlow lens is also recommended since it increases the magnification
and reveals any drift faster. When looking due south, insert the diagonal so the eyepiece points straight up. Insert the
cross hair eyepiece and align the cross hairs so that one is parallel to the declination axis and the other is parallel to the
right ascension axis. Move your telescope manually in R.A. and DEC to check parallelism.
First, choose your star near where the celestial equator and the meridian meet. The star should be approximately within
1/2 an hour of the meridian and within five degrees of the celestial equator. Center the star in the field of your
telescope and monitor the drift in declination.
Figure 6-6
The two stars in the front of the bowl of the Big Dipper point to Polaris which is less than
one degree from the true (north) celestial pole. Cassiopeia, the “W” shaped constellation,
is on the opposite side of the pole from the Big Dipper. The North Celestial Pole (N.C.P.)
is marked by the “+” sign.
Definition