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 two black nylon
alignment thumbscrews to redirect the aim of the finder
scope.
Note that the image seen through the finder scope appears
upside down. This is normal for astronomical finder scopes.
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.
When the target object is centered on the crosshairs of the
finder scope and in the telescope’s eyepiece, the finder scope
is now aligned and ready to be used for an observing session.
The finder scope and bracket can be removed from the dovetail
for storage, and then reinstalled without significantly changing
the finder scope’s alignment.
The finder scope alignment should be checked before every
observing session. This can easily be done at night, before
viewing through the telescope. Choose any bright star or
planet, center the object in the telescope eyepiece, and then
adjust the finder scope’s alignment screws until the star or
planet is also centered on the finder’s crosshairs. The finder
scope is an invaluable tool for finding objects in the night sky;
its usage for this purpose will be discussed later, in detail.
If, when looking through the finder scope, you notice that the
images appear somewhat out of focus, you will need to refo-
cus the finder scope for your eyes. First loosen the lock ring
located behind the objective lens cell on the body of the
finder scope (see Figure 3). Back the lock ring off by a few
turns, for now. Refocus the finder scope on a distant object by
threading the objective lens cell in or out of the finder scope
body. Precise focusing will be achieved by focusing the finder
scope on a bright star. Once the image appears sharp,
retighten the lock ring behind the objective lens cell. The
finder scope’s focus should not need to be adjusted again.
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 6) 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 (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 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.
7
Figure 6.
The equatorial mount of the AstroView 120ST EQ.
Declination
lock lever
Right Ascension
lock lever
Right Ascension
Setting Circle
Polar axis
finder scope
Latitude
adjusting T-bolts
Declination
axis
(Dec.)
Front opening
in R.A. axis
Latitude scale
Azimuth fine
adjustment
knobs
Right
ascension
(R.A.)
axis
Declination
Setting Circle