10
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 upper set
of numbers apply to viewing in the Northern Hemisphere,
while the numbers below them apply to viewing in the
Southern Hemisphere. The location of the R.A. coordinate
indicator arrow is shown in Figure 4.
The Dec. setting circle is scaled in degrees, with each mark
representing 2° increments. Values of Dec. coordinates range
from +90° to -90°. The 0° mark indicates the celestial equator.
When the telescope is pointed north of the celestial equator,
values of the Dec. setting circle are positive; when the tele-
scope is pointed south of the celestial equator, values of the
Dec. setting circle 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° 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 levers on the equatorial
mount, so the telescope optical tube can move freely.
3. Point the telescope at the bright star whose coordinates
you know. Lock the R.A. and Dec. lock levers. Center the star
in the telescope’s field of view with the slow-motion control
cables.
4. Loosen the R.A. setting circle lock thumbscrew (see Figure
4); this will allow the setting circle to rotate freely. Rotate the
setting circle until the arrow under the thumbscrew indicates
the R.A. coordinate listed in the star atlas for the object. Do
not retighten the thumbscrew when using the R.A. setting cir-
cles for finding objects; the thumbscrew is only needed for
polar alignment using the polar axis finder scope.
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 lever 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 tele-
scope is pointing south of the celestial equator. Retighten the
lock lever.
2. Loosen the R.A. lock lever 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 lever.
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 find-
er 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 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 cir-
cle 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 lat-
itude adjustment T-bolts. 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 lever
and rotate the telescope on the R.A. axis until the counter-
weight shaft is horizontal (parallel to the ground). Then loosen
the Dec. lock lever 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
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 9 illustrates how the telescope will look pointed at the
four cardinal directions—north, south, east, and west.
