9
Optional Motor Drives for Automatic tracking
An optional DC motor drive can be mounted on the R.A.
axis of the equatorial mount to provide hands-free tracking.
Objects will then remain stationary in the field of view without
any manual adjustment of the R.A. slow-motion control.
understanding the Setting Circles
The setting circles on an equatorial mount enable you to
locate celestial objects by their “celestial coordinates”. Every
object resides in a specific location on the “celestial sphere”.
That location is denoted by two numbers: its right ascension
(R.A.) and declination (Dec.). In the same way, every location
on Earth can be described by its longitude and latitude. R.A.
is similar to longitude on Earth, and Dec. is similar to latitude.
The R.A. and Dec. values for celestial objects can be found
in any star atlas or star catalog.
The R.A. setting circle is scaled in hours, from 1 through 24,
with small marks in between representing 10-minute incre-
ments (there are 60 minutes in 1 hour of R.A.). The lower
set of numbers (closest to the plastic R.A. gear cover) apply
to viewing in the Northern hemisphere, while the numbers
above them apply to viewing in the Southern hemisphere.
The Dec. setting circle is scaled in degrees, with each mark
representing 1° increments. Values of Dec. coordinates range
from +90° to -90°. For Northern hemisphere observers, use
the numbers on the setting circle that are closest to the
eastern horizon. The 0° mark indicates the celestial equator;
values north of the Dec. = 0° mark are positive, while values
south of the Dec. = 0° mark are negative.
So, the coordinates for the Orion Nebula listed in a star atlas
will look like this:
R.A. 5h 35.4m Dec. –5° 27'
That’s 5 hours and 35.4 minutes in right ascension, and -5
degrees and 27 arc-minutes in declination (there are 60 arc-
minutes in 1 degree of declination).
Before you can use the setting circles to locate objects, the
mount must be well polar aligned, and the R.A. setting circle
must be calibrated. The Dec. setting circle has been calibrat-
ed at the factory, and should read 90° when the telescope
optical tube is parallel with the R.A. axis.
Calibrating the right Ascension Setting Circle
1. Identify a bright star near the celestial equator (Dec. = 0°)
and look up its coordinates in a star atlas.
2. Loosen the R.A. and Dec. lock knobs on the equatorial
mount, so the telescope optical tube can move freely.
3. Point the telescope at the bright star near the celestial
equator whose coordinates you know. Lock the R.A. and
Dec. lock knobs. Center the star in the telescope’s field of
view with the slow-motion control cables
4. Loosen the R.A. setting circle lock thumb screw located
just above the R.A. setting circle pointer; this will allow
the setting circle to rotate freely. Rotate the setting circle
until the pointer indicates the R.A. coordinate listed in the
star atlas for the object. Retighten the thumb screw.
Finding Objects With the Setting Circles
Now that both setting circles are calibrated, look up in a star
atlas the coordinates of an object you wish to view.
1. Loosen the Dec. lock knob and rotate the telescope until
the Dec. value from the star atlas matches the reading on
the Dec. setting circle. Retighten the Dec. lock knob.
2. Loosen the R.A. lock knob and rotate the telescope until
the R.A. value from the star atlas matches the reading on
the R.A. setting circle. Retighten the lock knob.
Most setting circles are not accurate enough to put an object
dead-center in the telescope’s eyepiece, but they should
place the object somewhere within the field of view of the
finder scope, assuming the equatorial mount is accurately
polar-aligned. Use the slow-motion controls to center the
object in the finder scope, and it should appear in the tele-
scope’s field of view.
Confused About Pointing the telescope?
Beginners occasionally experience some confusion about
how to point the telescope overhead or in other directions. In
Figure 1 the telescope is pointed north, as it would be during
polar alignment. The counterweight shaft is oriented down-
ward. But it will not look like that when the telescope is point-
ed in other directions. Let’s say you want to view an object
that is directly overhead, at the zenith. how do you do it?
One thing you DO NOT do is make any adjustment to the
latitude adjustment T-bolt. That will nullify the mount’s polar
alignment. Remember, once the mount is polar-aligned, the
telescope should be moved only on the R.A. and Dec. axes.
To point the scope overhead, first loosen the R.A. lock knob
and rotate the telescope on the R.A. axis until the coun-
terweight shaft is horizontal (parallel to the ground). Then
loosen the Dec. lock knob and rotate the telescope until it
is pointing straight overhead. The counterweight shaft is still
horizontal. Then retighten both lock levers.
Similarly, to point the telescope directly south, the counter-
weight shaft should again be horizontal. Then you simply
rotate the scope on the Dec. axis until it points in the south
direction.
What if you need to aim the telescope directly north, but at
an object that is nearer to the horizon than Polaris? You can’t
do it with the counterweight down as pictured in Figure 1.
Again, you have to rotate the scope in R.A. so the counter-
weight shaft is positioned horizontally. Then rotate the scope
in Dec. so it points to where you want it near the horizon.
To point the telescope to the east or west, or in other direc-
tions, you rotate the telescope on its R.A. and Dec. axes.
Depending on the altitude of the object you want to observe,
the counterweight shaft will be oriented somewhere between
vertical and horizontal.
Figure 7 illustrates how the telescope will look pointed at the
four cardinal directions—north, south, east, and west
To point the telescope to the east or west, or in other direc-
tions, you rotate the telescope on its R.A. and Dec. axes.
Depending on the altitude of the object you want to observe,
