6
shaft remains horizontal even when you let go with both
hands
(Figure 4b)
.
3. Retighten the counterweight lock knobs. The telescope is
now balanced on the R.A. axis.
4. To balance the telescope on the Dec. axis, first tighten
the R.A. lock lever, with the counterweight shaft still in the
horizontal position.
5. With one hand on the telescope optical tube, loosen the
Dec. lock lever. The telescope should now be able to
rotate freely about the Dec. axis. Loosen the tube ring
clamps a few turns, until you can slide the telescope tube
forward and back inside the rings. Its often helpful to use
a slight twisting motion on the optical tube while you push
or pull on it.
(Figure 4c)
.
6. Position the telescope in the mounting rings so it remains
horizontal when you carefully let go with both hands. This
is the balance point for the optical tube with respect to the
Dec. axis.
(Figure 4d)
7. Retighten the tube ring clamps.
8. The telescope is now balanced on both axes. When you
loosen the lock lever on one or both axes and manually
point the telescope, it should move without resistance and
should not drift from where you point it.
6. Setting Up and Using the
Equatorial Mount
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
(Figure 5)
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 observ-
ing.
This is accomplished by slowly rotating the telescope on its
right ascension (R.A.) axis, using only the R.A. slow-motion
cable. But first the R.A. axis of the mount must be aligned with
the Earth’s rotational (polar) axis – a process called polar align-
ment.
Polar Alignment
For Northern Hemisphere observers, approximate polar align-
ment is achieved by pointing the mount’s R.A. axis at the North
Star, or Polaris. It lies within 1° of the north celestial 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 6)
. The two stars at the end of the
“bowl” of the Big Dipper point right 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).
For general visual observation, an approximate polar alignment
is sufficient.
1. Level the equatorial mount by adjusting the length of the
three tripod legs.
2. There are two latitude adjustment T-Bolts (see
Figure
2
); loosen one T-Bolt while tightening the other. By doing
this you will adjust the latitude of the mount. Continue
adjusting the mount until the pointer on the latitude scale
is set at the latitude of your observing site. If you don’t
know your latitude, consult a geographical atlas to find it.
For example, if your latitude is 35° North, set the pointer
to 35. The latitude setting should not have to be adjusted
again unless you move to a different viewing location
some distance away.
3. Loosen the Dec. lock lever and rotate the telescope’s
optical tube until it is parallel with the R.A. axis, as it is
in Figure 5. The pointer on the Dec. setting circle should
read 90°. Retighten the Dec. lock lever.
4. Move the tripod so the telescope tube and R.A. axis
point roughly at Polaris. If you cannot see Polaris directly
from your observing site, consult a compass and rotate
Figure 6.
To find Polaris in the night sky, look north and find the
Big Dipper. Extend an imaginary line from the two "Pointer Stars" in
the bowl of the Big Dipper. Go about five times the distance between
those stars and you'll reach Polaris, which lies within 1° of the north
celestial pole (NCP).
Big Dipper
(in Ursa Major)
Little Dipper
(in Ursa Minor)
N.C.P.
Pointer
Stars
Polaris
Cassiopeia
Figure 7.
The polar axis finder scope.
Eyepiece
focus ring
Focus lock
set-screw (3)
Objective
lens
Alignment
set-screw (3)