8
motion control cable is not needed for tracking. Objects will
appear to move faster at higher magnifications, because
the field of view is narrower.
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 with-
out any manual adjustment of the R.A. slow-motion control
cable.
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 mount’s R.A. setting circle is scaled in hours, from 1
through 24, with small hash marks in between representing
10-minute increments. The numbers closest to the R.A.
axis gear apply to viewing in the Southern Hemisphere,
while the numbers above them apply to viewing in the
Northern Hemisphere. The Dec. setting circle is scaled in
degrees, with each small hash mark representing 2.5°.
The Dec. setting circle is scaled in degrees, with each mark
representing 2.5° increments. Values of Dec. coordinates
range from +90° to -90°. The 0° mark indicates the celes-
tial equator. When the telescope is pointed north of the
celestial equator, values of the Dec. setting circle are posi-
tive, while when the telescope is pointed south of the
celestial equator, values of the Dec. setting circle are neg-
ative.
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 cir-
cle must be calibrated. The Dec. setting circle has been
permanently calibrated at the factory, and should read 90°
whenever the telescope optical tube is parallel with the
R.A. axis.
Calibrating the Right Ascension Setting Circle
1.
Identify a bright star in the sky 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 whose coordi-
nates 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.
Rotate the setting circle until the metal arrow indicates
the R.A. coordinate listed in the star atlas for the object.
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. Remember that values of the Dec.
setting circle are positive when the telescope is pointing
north of the celestial equator (Dec.= 0°), and negative
when the telescope is pointing south of the celestial equa-
tor. Retighten the 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. Remember to use the upper set of
numbers 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 accu-
rately polar aligned. Use the slow-motion controls to center
the object in the finder scope, and it should appear in the
telescope’s field of view.
The R.A. setting circle must be re-calibrated every time
you wish to locate a new object. Do so by calibrating the
setting circle for the centered object before moving on to
the next one.
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
downward. But it will not look like that when the telescope
is pointed 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 counterweight shaft is horizontal (parallel to the
ground). Then loosen the Dec. lock knob and rotate the tel-
escope until it is pointing straight overhead. The
counterweight shaft is still horizontal. Then retighten both
lock knobs.
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.
