Meade RCX400 Instruction Manual download pdf (Page 57)

Equatorial Alignment

In equator

ial (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 surround-

ing the Earth upon which all stars appear to be placed. This mapping system is simi-

lar 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. 49) 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. 49). The celestial equa-

tor (

2, Fig. 49) 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°).

J 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

arbitr

ily chosen to pass through the constellation P

egasus, a sort of cosmic

Greenwich mer

idian.

R.A. coordinates range from 0hr 0min 0sec to 23hr 59min

59sec.

There are 24 pr

imar

y lines of R.A., located at 15-deg

ree inter

als along

the celestial equator

Objects located fur

ther and fur

ther East of the zero R.A. grid

line (0hr 0min 0sec) carr

y higher R.A. coordinates.

J 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 celes-

tial equator (such as the the constellations of Orion, Virgo, and Aquarius) is said

to have a Declination of zero, shown as 0° 0' 0."

Setting Circles

Setting circles included with the RCX400 models permit the location of faint celestial

objects not easily found by direct visual observation. The R.A. circle (

Fig. 51) is locat-

ed on the top surface of the telescope’s drive base. The Declination circle (

Fig. 50) is

located at the top of the fork tine. With the telescope pointed at the North Celestial

Pole, the Dec. circle should read 90° (understood to mean +90°). Objects located

below the 0-0 line of the Dec. circle carry minus Declination coordinates. Each division

of the Dec. circle represents a 1° increment. The R.A. circle runs from 0

to (but not

including) 24

, and reads in increments of 5

min

Earth’s

Rotation

0 Dec.

South 

Celestial 

Pole

Right Ascension

Star

Celestial 

Equator

-90 Dec.

+90 Dec.

North 

Celestial 

Pole

(Vicinity 

of Polaris)

Fig. 49: 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. The

xample presented in this

section assumes that you are

performing an alignment pro-

cedure for the first time with

your telescope and therefore,

the "Telescope: Mount" option

does not need to be selected.

If the telescope is equatorially

mounted, y

ou must choose

the "Polar" option from the

Autostar II "Telescope Mount"

menu.

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Meade RCX400 Instruction Manual Page 57

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