6
to latitude. The R.A. and Dec. values for celestial
objects can be found in any star atlas or star catalog.
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 (the nega-
tive sign denotes south of the celestial equator). There
are 60 minutes in 1 hour of R.A, and 60 arc-minutes in 1
degree of declination.
The telescope’s R.A.setting circle is scaled in hours, from
1 through 24, with small hash marks in between repre-
senting 10-minute increments.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 1° increments.
Before you can use the setting circles to locate objects,
the mount must be well polar aligned, and the setting
circles must be calibrated.The declination setting circle
was calibrated 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 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. Center the star
in the telescope’s field of view.Lock the R.A.and Dec.
lock knobs.
4. Rotate the R.A.setting circle so the pointer indicates
the R.A. listed for the bright star in the star atlas.
Finding Objects With the Setting Circles
Now that both setting circles are calibrated, look up in
a star atlas or observing guide 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 lock
knob. Note: If the telescope is aimed south and the
Dec. setting circle pointer passes the 0° indicator,
the value on the Dec. setting circle becomes a neg-
ative number.
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 your finder scope’s field of view,
but they’ll get you close, assuming the equatorial mount
is accurately polar-aligned. The R.A. setting circle
should be recalibrated 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 mount overhead or in other direc-
tions. In Figure 1 the mount 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 undo 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 telescope until it is pointing straight over-
head. The counterweight shaft is still horizontal. Then
retighten both lock knobs.
Similarly, to point the telescope directly south, the
counterweight shaft should again be horizontal. Then
you simply rotate the scope on the Dec. axis until it
points in the south direction (Figure 5a).
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 that the counterweight shaft is positioned horizontal-
ly. Then rotate the scope in Dec. so it points to where
you want it near the horizon (Figure 5b).
To point the telescope to the east (Figure 5c) or west
(Figure 5d), or in other directions, you rotate the tele-
scope on its R.A. and Dec. axes. Depending on the
altitude of the object you want to observe, the counter-
weight shaft will be oriented somewhere between
vertical and horizontal.
The key things to remember when pointing the telescope
is that a) you only move it in R.A. and Dec., not in
azimuth or latitude (altitude), and b) the counterweight
and shaft will not always appear as it does in Figure 1.In
fact it almost never will!
