Meade ETX-90EC Astro Telescope Instruction Manual
POLAR ALIGNMENT
For extensive astronomical observing the telescope is best mounted in the polar
configuration. In polar alignment the telescope is oriented so that the horizontal
and vertical axes of the telescope are lined up with the celestial coordinate
system (see Fig. 10).
To polar align the ETX-90EC 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 polar-aligned astronomy, and
includes instructions for finding the celestial pole and for following objects in the
night sky using Declination and Right Ascension.
Fig. 12: Examples of AltAz and polar mounting of the ETX-
90EC to the optional #883 Deluxe Field Tripod.
Celestial Coordinates
Celestial objects are mapped according to a coordinate system on the Celestial Sphere (Fig. 13), 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. 13) 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. 13) 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
WARNING! Never use the
Meade ETX-90EC Astro
Telescope to look at the Sun!
Looking at or near the Sun
will
cause
instant
and
irreversible
damage to
your eye. Eye damage is often painless,
so there is no warning to the observer
that damage has occurred until it is too
late. Do not point the telescope or its
viewfinder at or near the Sun. Do not
look through the telescope or its
viewfinder as it is moving. Children
should always have adult supervision
while observing.
Fig. 13: Celestial Sphere.
