Appendix B: Equatorial Use; Celestial Coordinates; Lining Up With The Celestial Pole - Meade LX200 Instruction Manual

APPENDIX B: EQUATORIAL USE

1. Celestial Coordinates

Celestial objects are mapped according to a coordinate system
on the Celestial Sphere, an imaginary sphere surrounding
Earth on which all stars appear to be placed. This celestial
object mapping system is analogous to the Earth-based
coordinate system of latitude and longitude.
The poles of the celestial coordinate system are defined as
those two points where the Earth's rotational axis, if extended
to infinity, north and south, intersect the celestial sphere. Thus,
the North Celestial Pole (1, Fig. 20) is that point in the sky
where an extension of the Earth's axis through the North Pole
intersects the celestial sphere. This point in the sky is located
near the North Star, Polaris.
In mapping the surface of the Earth, lines of longitude are
drawn between the North and South Poles. Similarly, lines of
latitude are drawn in an east-west direction, parallel to the
Earth's Equator. The Celestial Equator (2, Fig. 20) is a
projection of the Earth's Equator onto the celestial sphere.
Just as on the surface of the Earth, in mapping the celestial
sphere, imaginary lines have been drawn to form a coordinate
grid. Thus, object positions on the Earth's surface are specified
by their latitude and longitude. For example, you could locate
Los Angeles, California, by its latitude (+34°) and longitude
(118°); similarly, you could locate the constellation Ursa Major
(which includes the Big Dipper) by its general position on the
celestial sphere:
R.A.: 11hr; Dec: +50°.
Right Ascension: The celestial analog to Earth longitude
•
is called "Right Ascension," or "R.A.," and is measured in
time on the 24 hour "clock" and shown in hours ("hr"),
minutes ("min") and seconds ("sec") from an arbitrarily
defined "zero" line of Right Ascension passing through the
constellation Pegasus. Right Ascension coordinates range
from 0hr 0min 0sec to 23hr 59min 59sec. Thus 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 prime Right Ascension grid line (0hr
0min 0sec) carry increasing R.A. coordinates.
• Declination: The celestial analog to Earth latitude is called
Declination, or "Dec", and is measured in degrees,
minutes and seconds (e.g., 15° 27' 33"). Declination
shown as north of the celestial equator is indicated with a
"+" sign in front of the measurement (e.g., the Declination
of the North Celestial Pole is +90°), with Declination south
of the celestial equator indicated with a "–" sign (e.g., the
Declination of the South Celestial Pole is –90°). Any point
on the celestial equator itself (which, for example, passes
through the constellations Orion, Virgo and Aquarius) is
specified as having a Dec of zero, shown as 0° 0' 0".
With all celestial objects therefore capable of being specified in
position by their celestial coordinates of Right Ascension and
Declination, the task of finding objects (in particular, faint
objects) is vastly simplified. The setting circles, R.A (10, Fig. 1)
and Dec. (3, Fig. 1) of the LX200 telescope may be dialed, in
North Celestial Pole
1
(Vicinity of Polaris)
Fig. 20: The Celestial Sphere.
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effect, to read the object coordinates and the object found
without resorting to visual location techniques. However, these
setting circles may be used to advantage only if the telescope
is first properly aligned with the North Celestial Pole.

2. 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 axial rotation). During
any 24 hour period, stars make one complete revolution about
the pole, making concentric circles with the pole at the center.
By lining up the telescope's polar axis with the North Celestial
Pole (or for observers located in Earth's Southern Hemisphere
with the South Celestial Pole (see MODE FUNCTIONS, page 16)
astronomical objects may be followed, or tracked, simply by
moving the telescope about one axis, the polar axis. In the case
of the Meade LX200 7", 8", 10", and 12" Schmidt-Cassegrain
telescopes, this tracking may be accomplished automatically
with the electric motor drive.
If the telescope is reasonably well aligned with the pole,
therefore, very little use of the telescope's Declination slow
motion control is necessary—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). 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's motor drive
will track accurately and keep objects in the telescopic field of
view for perhaps 20 to 30 minutes.
Begin polar aligning the telescope as soon as you can see
Polaris. Finding Polaris is simple. Most people recognize the
"Big Dipper." The Big Dipper has two stars that point the way to
Polaris (see Fig. 21). Once Polaris is found, it is a
straightforward procedure to obtain a rough polar alignment.
Little Dipper
Big Dipper
Fig. 21: Locating Polaris.
To line up the 7", 8", 10" or 12" LX200 with the Pole, follow this
procedure:
a. Using the bubble level located on the floor of the wedge,
adjust the tripod legs so that the telescope/ wedge/tripod
system reads "level."
b. Set the equatorial wedge to your observing latitude as
described in Appendix A.
c. Loosen the Dec. lock, and rotate the telescope tube in
Declination so that the telescope's Declination reads 90°.
Tighten the Dec. lock. Loosen the R.A. lock, and rotate the
Fork Arms to the 00 H.A. position (see MODE FUNCTIONS,
page 16) and initiate the POLAR align sequence on the
keypad.
d. Using the azimuth and latitude controls on the wedge,
center Polaris in the field of view. Do not use the
telescope's Declination or Right Ascension controls during
this process.
At this point, your polar alignment is good enough for casual
2
observations. There are times, however, when you will need to
have precise polar alignment, such as when making fine
astrophotographs or when using the setting circles to find new
objects.
Polaris
Cassiopeia