5
rotate the mount so the telescope points North. Retighten
the azimuth lock knob.
The equatorial mount is now polar-aligned for casual observ-
ing. Note that from this point on in your observing session,
you should not make any further adjustments in the azimuth
or the latitude of the mount, nor should you move the tripod.
Doing so will nullify the polar alignment. The telescope should
be moved only about its R.A. and Dec. axes.
tracking celestial objects
When you observe a celestial object through the telescope,
you’ll see it drift slowly across the field of view. To keep it in
the field, if your equatorial mount is polar aligned, just turn the
R.A. slow-motion control. The Dec. slow-motion control is not
needed for tracking. Objects will appear to move faster at
higher magnifications, because the field of view is narrower.
understanding the setting circles
The setting circles on an equatorial mount enable you to
locate celestial objects by their “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 hash marks in between representing 10-minute
increments (there are 60 minutes in 1 hour of R.A.). The num-
bers closest to the R.A. 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
degrees (there are 60 arc-minutes in 1 degree of declination).
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 negative sign
denotes south of the celestial equator).
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 is already perma-
nently calibrated at the factory).
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 bolts 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. This information can
be taken from any star chart. Center the star in the tele-
scope’s field of view. Retighten the R.A. and Dec. lock bolts.
4. Rotate the R.A. setting circle so the pointer indicates the
R.A. listed for that object in the star atlas.
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 bolt and rotate the telescope until
the Dec. value from the star atlas matches the reading on
the Dec. setting circle. Retighten the lock bolt.
2. Loosen the R.A. lock bolt 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 bolt.
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 must 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 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
latitude adjustment knob. 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 bolt
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 bolt and rotate the telescope until it is pointing
straight overhead. The counterweight shaft is still horizontal.
Then retighten both lock bolts.
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.
To point the telescope to the east or west, or in other directions,
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.
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!
