Bedienungsanleitung Meade ETX-105AT (Seite 51 von 60) (Englisch)

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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.

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 finding the celestial pole and for finding 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 defined as those two points where the Earth’s North and

South poles (

i.e., the Earth's axis), if extended to infinity, would cross the celestial sphere.

Thus, the North Celestial Pole (

1, Fig. 42) 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 (

1, Fig. 42). The celestial equator (2, Fig. 42) 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 (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 merid-

ian. 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.

■ 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."

Polar Mounting your Telescope with the #884 Deluxe Field Tripod

As the Earth rotates on its axis, astronomical objects appear to move across the sky in an

arc. This apparent motion is not obvious to the unaided eye, but when viewed through a

telescope such as an ETX, this motion is rapid indeed. If the motor drive has not been

engaged, celestial objects viewed in the telescope eyepiece drift

entirely out of the field of view in 15 to 60 seconds, depending on the magnification

employed.

However, by mounting your telescope and turning on the motor drive, you can

counteract the drift of objects in the eyepiece. The two basic types of mounting are:

altazimuth (alt/az) and equatorial. For many ETX owners, it will not be necessary to

equatorially mount the telescope; alt/az mounting will suffice. The Autostar Controller allows

the telescope to be mounted in the altazimuth (alt/az) mode for most

observing purposes. See

EASY (TWO-STAR) ALIGN, page 19, for more information.

Earth’s

Rotation

0 Dec.

South 

Celestial 

Pole

Right Ascension

Star

Celestial 

Equator

-90 Dec.

+90 Dec.

North 

Celestial 

Pole

(Vicinity 

of Polaris)

Fig. 42: Celestial Sphere.

APPENDIX A: EQUATORIAL

(POLAR) ALIGNMENT

Important Note:

The "Telescope: Mount"

option of the Setup menu is

set to "Alt/Az" as the default

mount by the factory. If the

telescope is equatorially

mounted, you must choose

the "Polar" option from the

Autostar "Telescope: Mount"

menu.

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