13
Once this line of axial movement has been
established, set the star at the edge of the
field and allow it to drift across via its own
motion. If the star drifts upwards from this
line (figure 5a ), then the north end of the
polar axis needs to be raised. If the star drifts
below this line (figure 5b ), the north end of
the polar axis needs to be lowered.
This altitude adjustment is shown in f i g u re 1a .
Polar axis
(azimuth) alignment
Now observe a star in the south and repeat
the preceding observation exercise. This
time, an upward drift (figure 5a ) indicates
that the north end of the polar axis should be
shifted west; if the star drops (figure 5b ), the
shift should be east. This azimuth adjustment
is shown in figure 1b .
It may be necessary to alternate a few times
between these two alignment exercises. Once
these adjustments have been completed suc-
c e s s f u l l y, a star allowed to drift across the field
should run parallel to the line established by
manual axial movement, to point c in f i g u re 5.
(As shown with the reticles in f i g u re 4.)
It cannot be emphasised enough that the
additional adjustments made should be slight
and as the correct alignment is approached
the tell tale deviations will become harder to
detect.
Having aligned the telescope mount with the
E a rth's polar axis it now becomes possible to
use the sky's co-ordinate system to find objects
not visible to the eye or finder scope.
The setting circles
The sky is plotted in a similar way to the
Earth's longitude and latitude system, except
that the celestial equivalents are called Right
Ascension (R.A.) and Declination respectively.
Right Ascension is divided into 24 'hours',
each subdivided into minutes and seconds.
Declination is measured in degrees, com-
mencing with +90° at the north celestial pole
(towards which the telescope's polar axis
points, for northern observers) decreasing to
0° at the celestial equator.
Below the celestial equator, declinations are
designated as minus, rising from 0° to -90° at
the south celestial pole.
Calibrating the declination circle.
In order to use the equatorial mount's
d e clination scale eff e c t i v e l y, the declination
c i rcle needs to be fixed so that pointer indi-
cates +90° when the telescope is aimed at
the celestial pole.
Use the telescope at its highest power to
observe a star whose declination is known.
(All good star atlases should have this
i n f o rmation available. In lieu of such a
s o u rce, a list of bright stars easily found in
a star map appears in the appendix.)
Keeping the star in the centre of the eyepiece
field, loosen the locking mechanism of the
declination circle. Turn the declination circle
until the star's correct declination is indexed
by the pointer and lock the circle. There
should be no need to repeat this exercise - so
long as polar axis alignment is maintained.
axial motion line
Figure 5
a
c
b
