4
be able to rotate freely about the Dec. axis. If the front of
the telescope swings downward, that means you need to
move it back in the mount’s saddle. If the front of the tele-
scope swings upward, then you need to shift the telescope
forward in the saddle. To move the telescope in the saddle,
loosen the saddle lock knobs just a little – so the dovetail
bar doesn’t accidentally pop out of the saddle. Position the
telescope so it remains horizontal when you carefully let go
with both hands. This is the balance point.
6. Retighten the saddle clamp knobs.
The telescope is now balanced on both axes. Now when you
loosen the lock knob on one or both axes and manually point
the telescope, it should move without resistance and should
not drift from where you point it.
V. Polar Alignment
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 apparent motion is 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.
This is accomplished by slowly rotating the telescope on its
right ascension (R.A.) axis, using only the R.A. slow-motion
knob. 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 observers, approximate polar align-
ment is achieved by pointing the mount’s right ascension axis
at the North Star, Star (Polaris). It lies within 1° of the north
celestial pole (NCP), which is an extension of the Earth’s rota-
tional 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 8
). The two stars at the end of the
“bowl” of the Big Dipper point approximately to 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).
To polar align the equatorial mount:
1. Roughly level the mount by adjusting the length of the three
tripod legs as needed (
Figure 9
).
Figure 4. A
) Thread the counterweight shaft onto the mount.
B
) After removing the safety stop, slide the counterweight onto the
shaft.
Figure 5.
Attach the two slow-motion cables to the mount.
Figure 6.
The assembled EQ-13 mount looks like this.
A.
B.
Safety stop
Dec. slow-motion
cable
R.A. slow-motion
cable
Saddle
clamp
knobs
Dec. lock knob
R.A. lock knob
Dovetail
saddle
R.
A.
(P
olar) axis
Dec.
axis
To P
olar
is
Slow motion
cable
