Meade Instruments LX600 - Manual (Page 57)

57
Appendix E
APPENDIX E: EQUATORIAL
(POLAR) ALIGNMENT
Equatorial Alignment
In equatorial (or “polar”) alignment, the telescope is oriented so
that the horizontal and vertical axes of the telescope are lined up
with the celestial coordinate system.
Important Note: The “Telescope: Mount”option of the Setup
menu is set to “Alt-az” as the default mount by the factory.
The example presented in this section assumes that you
are performing an alignment procedure for the fi rst time
with your telescope and therefore, the “Telescope: Mount”
option does not need to be selected.
If the telescope is equatorially mounted, you must choose the
“Polar” option from the AutoStar II “Telescope Mount” menu.
In order to equatorial align your telescope, it is essential to have
an understanding of how and where to locate celestial objects
as they move across the sky. This section provides a basic
introduction to the terminology of equatorial-aligned astronomy,
and includes instructions for fi nding the celestial pole and
for fi nding objects in the night sky using Declination and
Right Ascension.
Celestial Coordinates
A celestial coordinate system was created that maps an imaginary
sphere surrounding the Earth upon which all stars appear to be
placed. This mapping system is similar to the system of latitude
and longitude on Earth surface maps.
In mapping the surface of the Earth, lines of longitude are drawn
between the North and South Poles and lines of latitude are
drawn in an East-West direction, parallel to the Earth’s equator.
Similarly, imaginary lines have been drawn to form a latitude and
longitude grid for the celestial sphere. These lines are known as
Right Ascension and Declination.
The celestial map also contains two poles and an equator just
like a map of the Earth. The poles of this coordinate system are
defi ned as those two points where the Earth’s North and South
poles (i.e., the Earth’s axis), if extended to infi nity, would cross
the celestial sphere. Thus, the North Celestial Pole (Fig. 19, 1)
is that point in the sky where an extension of the North Pole
intersects the celestial sphere. The North Star, Polaris, is located
very near the North Celestial Pole (Fig. 19, 1). The celestial
equator (Fig. 19, 2) is a projection of the Earth’s equator onto the
celestial sphere.
So just as an object’s position on the Earth’s surface can be
located by its latitude and longitude, celestial objects may also
be located using Right Ascension and Declination. For example:
You could locate Los Angeles, California, by its latitude (+34°)
and longitude (118°). Similarly, you could locate the Ring Nebula
(M57) by its Right Ascension (18hr) and its Declination (+33°).
■ Right Ascension (RA): 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. RA coordinates range
from 0hr 0min 0sec to 23hr 59min 59sec.
There are 24 primary lines of RA, located at
15-degree intervals along the celestial equator.
Objects located further and further East of the
zero RA grid line (0hr 0min 0sec) carry higher
RA coordinates.
■ 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".
Fig. 19. Celestial Sphere.