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, "Dec", and is measured in
degrees, minutes and seconds (e.g., 15° 27' 33"). Declination shown as north of the celestial equator is
indicated with a "+" sign in front of the measurement (e.g., the Declination of the North Celestial Pole is
+90°), with Declination south of the celestial equator indicated with a "-" sign (e.g., the Declination of the
South Celestial Pole is -90°). Any point on the celestial equator itself (which, for example, passes through
the constellations Orion, Virgo and Aquarius) is specified as having a Declination of zero, shown as 0° 0'
0".
All celestial objects are specified in position by their celestial coordinates of Right Ascension and Declination.
Locating the Celestial Pole
To get basic bearings at an observing location, take note of where the sun rises (East) and sets (West) each day.
After the site is dark, face north by pointing your left shoulder toward where the sun set. To precisely point at the
pole, find the North Star (Polaris) by using the Big Dipper as a guide (Fig. 16).
Polar Alignment Procedure
As the Earth rotates once on its axis every 24 hours, astronomical objects appear to move across the sky in an arc.
This apparent motion (see
Sidereal Rate
) is not obvious to the unaided eye, but viewed through a serious
telescope such as the ETX-90EC, this motion is rapid indeed. If the motor drive has not been engaged, objects
centered in the telescope's eyepiece move entirely out of the field of view in 30 to 160 seconds, depending on the
magnification employed.
For easy tracking of astronomical objects the ETX-90EC should be polar aligned.
There are two mounting methods available to polar align the ETX-90EC: by use of the optional
#883 Deluxe Field
Tripod
or the
#880 Table Tripod
(see
OPTIONAL ACCESSORIES
).
To Polar align using the
#883 Deluxe Field Tripod
(Fig. 12), follow the instructions provided with the tripod. To Polar
align using the
#880 Table Tripod
, follow the procedure below.
1. Make sure the viewfinder is aligned with the ETX-90EC (see
Aligning the Viewfinder
).
2. Remove the two hole covers (13, Fig. 1) from the side of the drive base and thread the two identical fixed
legs (4, Fig. 15) into these holes to a firm feel only.
3. Determine the latitude of the observing location from a road map, atlas, or from the
Latitude Chart for Major
Cities of the World
; determining the latitude within about one degree is sufficient.
4. The
#880 Table Tripod
includes two adjustable tripod legs: The standard tripod leg is used at observing
latitudes between 22° and 48.5° and has a dual latitude label attached (Fig. 14). The high-latitude tripod leg
is shorter and is used at observing latitudes between 44° and 66°. Based on the observing latitude
determined in step 3, set aside the tripod leg that is not to be used.
5. Two mounting holes are located on the bottom of the telescope drive base. Mount the appropriate
adjustable tripod leg (as determined in step 4) to the drive base using the following latitudes:
Standard Tripod Leg
o
32.5° to 48.5° uses High-Latitude hole (2, Fig. 15).
o
22° to 35.5° uses Alternate hole (3, Fig. 15).
