Leisure Time LX20 - Manual (Page 29)

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.: 11 hr; 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 Ohr Omin
Osec 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 (Ohr Omin Osec) 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
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.
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
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.
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