7
Calibrating the Declination Setting Circle
1. Loosen the Dec. lock lever and position the telescope as
accurately as possible in declination so it is parallel to the
R.A. axis of the equatorial mount. Re-tighten the lock lever.
2. Rotate the Dec. setting circle until the pointer reads exactly 90°.
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 levers 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. Lock the R.A. and Dec. lock levers.
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 lever and rotate the telescope until
the Dec. value from the star atlas matches the reading on
the Dec. setting circle. 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. Retighten the lock lever.
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 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 1. Again,
you have to rotate the scope in R.A. so that 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 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.
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!
6. using Your telescope—
Astronomical Observing
Choosing an Observing Site
When selecting a location for observing, get as far away as
possible from direct artificial light such as streetlights, porch
lights, and automobile headlights. The glare from these lights
will greatly impair your dark-adapted night vision. Set up on a
grass or dirt surface, not asphalt, because asphalt radiates
more heat. Heat disturbs the surrounding air and degrades
the images seen through the telescope. Avoid viewing over
rooftops and chimneys, as they often have warm air currents
rising from them. Similarly, avoid observing from indoors
through an open (or closed) window, because the tempera-
ture difference between the indoor and outdoor air will cause
image blurring and distortion.
If at all possible, escape the light-polluted city sky and head
for darker country skies. You’ll be amazed at how many more
stars and deep-sky objects are visible in a dark sky!
Cooling the telescope
All optical instruments need time to reach “thermal equilib-
rium.” The bigger the instrument and the larger the temperature
change, the more time is needed. Allow at least a half-hour for
your telescope to cool to the temperature outdoors. In very
cold climates (below freezing), it is essential to store the tele-
scope as cold as possible. If it has to adjust to more than a
40° temperature change, allow at least one hour.
Aiming the telescope
To view an object in the main telescope, first loosen both the R.A.
and Dec. lock levers. Aim the telescope at the object you wish to
observe by “eyeballing” along the length of the telescope tube (or
use the setting circles to “dial in” the object’s coordinates). Then
look through the (aligned) finder scope and move the telescope
tube until the object is centered on the crosshairs. Retighten the
R.A. and Dec. lock levers. Then center the object on the finder’s
crosshairs using the R.A. and Dec. slow-motion controls. The
object should now be visible in the main telescope with a low-
power (long focal length) eyepiece.
