7
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 perma-
nently 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 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.
Celestial equator (Dec. = 0°)
Dec. coordinate indicator arrow
Use these numbers on
the R.A. setting circle
(Northern Hemisphere)
Figure 5: For Northern Hemisphere observers, use the
lower set of numbers on the R.A. setting circle. The R.A.
coordinate indicator arrow is between the “R” and the
“A” on the plastic gear cover. The telescope (not shown)
is pointing north of the celestial equator, so the
approximate coordinates of the mount in the image are
R.A. 8h 30m Dec. 60°.
Plastic R.A.
gear cover
R.A. coordinate
indicator arrow
4. Loosen the R.A. setting circle lock thumb screw (see Figure
2); this will allow the setting circle to rotate freely. Rotate the
setting circle until the arrow between the “R” and the “A” on
the plastic gear cover 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. 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 lower set of num-
bers 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 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.
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 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 lati-
tude 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 counter-
weight 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 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.
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 counterweight
shaft is positioned horizontally. Then rotate the scope in Dec.
so it points to where you want it near the horizon.
