6
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 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.). On the
R.A. setting circle there are two sets of numbers. The numbers
farthest from the R.A. gear apply to viewing in the Northern
Hemisphere, while the numbers below them apply to viewing in
the Southern Hemisphere. The Dec. setting circle is scaled in
degrees (there are 60 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 (5 degrees south of the celestial equator) and 27
minutes in declination.
Before you can use the setting circles to locate objects, they
must first be calibrated. The declination setting circle was cal-
ibrated at the factory, and should read 90° when the
telescope optical tube is pointing exactly along the polar axis.
If it does not read 90°, it may have to be reset.
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 (not the azimuth lock knob or latitude adjustment
knob), 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 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 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.
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 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 lat-
itude adjustment knob. That will spoil 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.
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.
You get the idea. The key things to remember when pointing the tel-
escope 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).
6. Collimating the Optics
(Aligning the Mirrors)
Collimation is the process of adjusting the mirrors so that they
are perfectly aligned with each other. Your telescope’s optics
were aligned at the factory, and should not need adjustment
unless the telescope was roughly handled during shipment.
Accurate alignment is important to insuring the peak perform-
ance of your telescope, so it should be checked occasionally.
Collimation is easy to do and should be done in daylight.
To check the collimation, remove the eyepiece and look down
the focuser drawtube. You should see the secondary mirror
centered in the drawtube, the reflection of the primary mirror
centered in the secondary mirror, and the reflection of the
