Orion SKYVIEW PRO 120 EQ Instruction Manual page 11

Dec. setting
circle
Dec. setting
circle
thumbscrew
(2)
Dec.
indicator
arrow
R.A.
indicator
arrow
Figure 11.
The R.A. and Dec. setting circles.
ments (there are 60 minutes in 1 hour of right ascension).
The lower set of numbers apply to viewing in the Northern
Hemisphere, while the numbers above them apply to viewing
in the Southern Hemisphere.
The Dec. setting circle is scaled in degrees, with each mark
representing 2° increments. Values of declination coordinates
range from +90° to -90°. The 0° mark indicates the celestial
equator. When the telescope is pointed north of the celestial
equator, values of the declination setting circle are positive;
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
(see Figure 11), 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 equa-
tor (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
R.A.
setting
circle
R.A. setting circle
thumbscrew (2)
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.
Loosen the Dec. lock lever and rotate the telescope until the
declination value from the star atlas matches the reading on
the Dec. setting circle. Remember that values of the Dec. set-
ting 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.
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 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?
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
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