Meade ETX-90PE instruction manual Equatorial Alignment, Celestial Coordinates

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.

North
Celestial				+90		Dec.
Pole								Star
(Vicinity
of Polaris)							e
						D
							c
1							l
							a
							i
							n
								t
								i
		14	13	12		11	10	o
	15							n
17 16								9	8
	Earth’s									7
18	Rotation									6
19									4	5
20	21						2	3		Celestial
		22	23		0	1
										Equator
Right Ascension
										0 Dec.
South										2
Celestial				-	90	Dec.
Pole

Fig. 43: Celestial Sphere.

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APPENDIX A: 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.

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. 43) is that point in the sky where an exten- sion of the North Pole intersects the celestial sphere. The North Star, Polaris, is located very near the North Celestial Pole (1, Fig. 43). The celestial equator (2, Fig. 43) is a pro- jection 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°).

JRight 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.

JDeclination (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 AUTOMATIC ALIGNMENT, page 17, for more information.