when the telescope is pointed south of the celestial equator,
values of the declination setting circle are negative.
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 (there are 60 arc-
minutes in 1 degree of declination).
Before you can use the setting circles to locate objects, the
mount must be accurately polar aligned, and the setting cir-
cles must be calibrated.
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 as shown in Figure 1. Re-tighten the lock lever.
2. Loosen one of the thumbscrews on the Dec. setting circle,
this will allow the setting circle to rotate freely. Rotate the
Dec. setting circle until the pointer reads exactly 90°. Re-
tighten the setting circle thumbscrew.
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 slow-motion
control knobs.
4. Loosen one of the R.A. setting circle thumbscrews (see
Figure 11); 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 point-
ing north of the celestial equator (Dec. = 0°), and negative
when the telescope is pointing south of the celestial equa-
tor. 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 slow-motion controls to center the object in
the finder scope, and it should appear in the telescope’s field
of view.
The setting circles must 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 over-
head, first loosen the R.A. lock lever and rotate the telescope
on the right ascension axis until the counterweight shaft is hor-
izontal (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 counterweight 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.
Figure 11.
The R.A. and Dec. setting circles.
11
Dec.
setting circle
Dec.
setting circle
thumbscrew
(2)
Dec.
indicator
arrow
R.A.
indicator
arrow
R.A. setting circle
thumbscrew (2)
R.A.
setting circle
