Calibrating the Right Ascension Setting Circle
1. Identify a bright star in the sky near the celestial equator
(declination = 0°) 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 whose coordinates
you know. Lock the R.A. and Dec. lock levers. Center the
star in the telescope’s field of view with the hand con-
troller.
4. Loosen one of the R.A. setting circle thumbscrews (see
Figure 12) this will allow the setting circle to rotate freely.
Rotate the setting circle until the R.A. pointer arrow indi-
cates the R.A. coordinate listed in the star atlas for the
object. Re-tighten the setting circle thumbscrew.
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 declination value from the star atlas matches the read-
ing 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 equator. Retighten the lock lever.
2. Loosen the R.A. lock lever and rotate the telescope until
the right ascension value from the star atlas matches the
reading on the R.A. setting circle. Remember to use the
lower set of numbers on the R.A. setting circle. Retighten
the lock lever.
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 hand controller to center the object in the
finder scope, and it should appear in the telescope’s field of
view.
The setting circles should be re-calibrated every time you
wish to locate a new object. Do so by calibrating the setting
circles 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 point-
ed in other directions. Let’s say you want to view an object
that is directly overhead, at the zenith. How do you do it?
DO NOT make any adjustment to the latitude adjustment L-
bolts. 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 lever and rotate the tele-
scope on the right ascension axis until the counterweight
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.
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 counterweights down as pictured in Figure 1. Again,
you have to rotate the scope in right ascension so that the
counterweight shaft is positioned horizontally. Then rotate the
scope in declination so it points to where you want it near the
horizon.
To point the telescope directly south, the counterweight shaft
should again be horizontal. Then you simply rotate the scope
on the declination axis until it points in the south direction.
To point the telescope to the east or west, or in other direc-
tions, you rotate the telescope on its right ascension and dec-
lination axes. Depending on the altitude of the object you want
to observe, the counterweight shaft will be oriented some-
where between vertical and horizontal.
Figure 13 illustrates how the telescope will look when pointed
at the four cardinal directions: north, south, east and west.
The key things to remember when pointing the telescope are
that a) you only move it in right ascension and declination, 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!
Figure 13a-d.
These illustrations show the telescope pointed in the four cardinal directions. (a) north, (b) south, (c) east, (d) west. Note
that the tripod and mount have not been moved; only the telescope has been moved on the its R.A. and Dec. axes.
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a.
b.
c.
d.
