5
3. Balancing the telescope
To insure smooth movement of the telescope on both axes of
the equatorial mount, it is imperative that the optical tube be
properly balanced. We will first balance the telescope with
respect to the R.A. axis, then the Dec. axis.
1. Keeping one hand on the telescope optical tube, loosen
the R.A. lock lever. Make sure the Dec. lock lever is locked,
for now. The telescope should now be able to rotate freely
about the R.A. axis. Rotate it until the counterweight shaft
is parallel to the ground (i.e., horizontal).
2. Now loosen the counterweight lock knob and slide the
weight along the shaft until it exactly counterbalances the
telescope. That’s the point at which the shaft remains
horizontal even when you let go with both hands.
3. Retighten the counterweight lock knob. 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 (this can be aided by using a
slight twisting motion on the optical tube while you push or
pull on it).
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.
7. Retighten the tube ring clamps.
The telescope is now balanced on both axes. Now 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.
4. Aligning the Finder Scope
A finder scope has a wide field of view to facilitate the location
of objects for subsequent viewing through the main tele-
scope, which has a much narrower field of view. The finder
scope and the main telescope must be aligned so they point
to exactly the same spot in the sky.
Alignment is easiest to do in daylight hours. First, insert the
lowest-power (25mm) eyepiece into the star diagonal. Then
loosen the R.A. and Dec. lock levers so the telescope can be
moved freely.
Point the main telescope at a discrete object such as the top
of a telephone pole or a street sign that is at least a quarter-
mile away. Move the telescope so the target object appears in
the very center of the field of view when you look into the
eyepiece. Now tighten the R.A. and Dec. lock levers. Use the
slow-motion control knobs to re-center the object in the field of
view, if it moved off center when you tightened the lock levers.
Now look through the finder scope. Is the object centered in
the finder scope’s field of view, i.e., on the crosshairs? If not,
hopefully it will be visible somewhere in the field of view, so
that only fine adjustment of the finder scope alignment screws
will be needed to center it on the crosshairs. Otherwise you’ll
have to make coarser adjustments to the alignment screws to
redirect the aim of the finder scope.
Once the target object is centered on the crosshairs of the
finder scope, look again in the main telescope’s eyepiece and
see if it is still centered there as well. If it isn’t, repeat the
entire process, making sure not to move the main telescope
while adjusting the alignment of the finder scope.
Note that the image seen through the finder scope
appears upside down. This is normal for astronomical
finder scopes.
5. 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 2, page 11) is designed to
compensate for that motion, allowing you to easily “track” the
movement of astronomical objects, thereby keeping them from
drifting out of the telescope’s field of view while you’re observing.
This is accomplished by slowly rotating the telescope on its right
ascension (polar) 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 alignment.
Polar Alignment
For Northern Hemisphere observers, approximate polar
alignment 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 Polaris.
To find Polaris in the sky, look north and locate the pattern of
the Big Dipper (Figure 3, page 11). 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 align-
ment is sufficient.
1. Level the equatorial mount by adjusting the length of the
three tripod legs.
2. Loosen the latitude lock lever. Turn the latitude adjustment
knob and tilt 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. Then retighten the latitude lock lever. The
