Meade ETX-80AT-TC instruction manual Appendix B Equatorial Polar Alignment, Celestial Coordinates

APPENDIX B: EQUATORIAL (POLAR) ALIGNMENT

Polar Alignment

The great majority of ETX-80AT owners will find it unnecessary ever to Polar align the telescope. With these ETX telescope models the standard-equip- ment Autostar controller allows the telescope to be used in the altazimuth (Alt/Az) orientation for all observing purpose. This section is included only for educational purposes, where the observer might wish to operate the analog setting circles (Fig. 28 and Fig. 29) in place of the digital setting circles built into the Autostar hand controller. Absent a desire to use the analog circles or simply to be informed about the use of the equatorial (Polar- aligned) mount, reading of this appendix may be omitted.

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.

2

South

Celestial

Pole -90 Dec.

Fig. 30: Celestial Sphere.

Fig. 28: Declination setting circle.

Fig. 29: Right Ascension setting circle.

In Polar Alignment, the telescope is oriented so that the horizontal and vertical axes of the tele- scope are lined up with the celestial coordinate system. Polar Alignment requires the telescope to be mounted to the optional #884 Deluxe Field Tripod .

In order to Polar 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 introduc- tion to the terminology of Polar-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

Celestial objects are mapped according to a coordinate system on the Celestial Sphere (Fig. 30), 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. 30) 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. 30) is a projection of the Earth’s Equator onto the celes- tial 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 lati- tude (+34°) and longitude (118° West); 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 or “hr," minutes or “min," and seconds or “sec," from an arbitrarily defined “zero” line of Right Ascension passing through the constellation Pegasus. Right Ascension coordinates range from 0hr 0min 0sec 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, 0hr 0min 0sec, carry increasing R.A. coordinates.

•Declination: The celestial analog to Earth latitude is called Declination, or “Dec,” and is measured in degrees, arc-minutes and arc-seconds, e.g., 15° 27' 33". Declination shown as North of the celestial equator is indicated with a “+” sign in front of the measurement,

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