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– 8 –
(also known as “M57”) by its Right Ascension (18hr) and its
Declination (+33°).
■ Right Ascension (R.A.): This celestial version of longitude is
measured in units of hours (hr), minutes (min), and seconds
(sec) on a 24-hour "clock" (similar to how Earth's time zones
are determined by longitude lines). The "zero" line was
arbitrarily chosen to pass through the constellation Pegasus, a
sort of cosmic Greenwich meridian. R.A. coordinates range
from 0hr 0min 0sec to 23hr 59min 59sec. There are 24 primary
lines of R.A., located at 15-degree intervals along the celestial
equator. Objects located further and further East of the zero
R.A. grid line (0hr 0min 0sec) carry higher R.A. coordinates.
See
Fig. 8.
■ Declination (Dec.): This celestial version of latitude is
measured in degrees, arc-minutes, and arc-seconds (
e.g., 15°
27' 33"). Dec. locations North of the celestial equator are
indicated with a plus (+) sign (
e.g., the Dec. of the North
celestial pole is +90°). Dec. locations South of the celestial equator are indicated with a minus (–) sign
(e.g., the Dec. of the South celestial pole is –90°). Any point on the celestial equator (such as the the
constellations of Orion, Virgo, and Aquarius) is said to have a Declination of zero, shown as 0° 0' 0." See
Fig. 8.
As all celestial objects therefore may be located with their celestial coordinates of Right Ascension and
Declination, the task of finding objects (in particular, faint objects) in the telescope is vastly simplified. The
setting circles, R.A (
34, Fig. 2) and Dec. (25, Fig. 2) of your telescope may be dialed, in effect, to read the
object coordinates and the object found without resorting to visual location techniques. However, these
setting circles only perform correctly if the telescope is properly aligned with the North Celestial Pole.
LINING UP WITH THE CELESTIAL POLE
Objects in the sky appear to revolve around the celestial
pole. (Actually, celestial objects are essentially “fixed,”
and their apparent motion is caused by the Earth’s
rotation). During any 24 hour period, stars make one
complete revolution about the pole, circling with the pole
at the center. By lining up the telescope’s polar axis (
40,
Fig. 2) with the North Celestial Pole (or for observers
located in Earth’s Southern Hemisphere with the South
Celestial Pole), astronomical objects may be followed, or
“tracked,” by moving the telescope about one axis, the
polar axis.
If the telescope is reasonably well aligned with the pole, therefore, very little use of the telescope’s
Declination flexible cable control is necessary and virtually all of the required telescope tracking will be in
Right Ascension. (If the telescope were perfectly aligned with the pole, no Declination tracking of stellar
objects would be required whatsoever). For the purposes of casual visual telescopic observations, lining up
the telescope’s polar axis to within a degree or two of the pole is more than sufficient: with this level of
pointing accuracy, the telescope can track accurately by slowly turning the telescope’s R.A. flexible cable
control and keep objects in the telescopic field of view for perhaps 20 to 30 minutes.
POLAR ALIGNMENT OF THE EQUATORIAL MOUNT
To line up the Meade 60EQ-A with the pole, follow this procedure:
1. Release the Azimuth lock (32, Fig. 1) of the Azimuth base, so that the entire telescope-with-mounting
may be rotated in a horizontal direction. Rotate the telescope until it points due North. Use a compass
or locate Polaris, the North Star (see
Fig. 9), as an accurate reference for due North.
2. Level the mount, if necessary, by adjusting the heights of the three tripod legs.
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0
1
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Rotation
of the
Earth
0°Dec.
South
Celestial
Pole
Right
Ascension
Star
Celestial
Equator
-90°Dec.
+90°Déc.
D
e
c
l
i
n
a
t
i
o
n
North
Celestial
Pole
(Vicinity of
Polaris)
Fig. 8: Celestial Sphere.
Polaris
Little Dipper
Big Dipper
Cassiopeia
Figure 9: Finding Polaris