For extensive astronomical observing the telescope is best mounted in the polar configuration. In polar alignment the telescope is oriented so that the horizontal and vertical axes of the telescope are lined up with the celestial coordinate system (see Fig. 10).
|WARNING! Never use the Meade ETX-90EC Astro Telescope to look at the Sun! Looking at or near the Sun will cause instant and irreversible damage to your eye. Eye damage is often painless, so there is no warning to the observer that damage has occurred until it is too late. Do not point the telescope or its viewfinder at or near the Sun. Do not look through the telescope or its viewfinder as it is moving. Children should always have adult supervision while observing. |
To polar align the ETX-90EC 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 polar-aligned astronomy, and includes instructions for finding the celestial pole and for following objects in the night sky using Declination and Right Ascension.
Fig. 12: Examples of AltAz and polar mounting of the ETX-90EC to the optional #883 Deluxe Field Tripod.
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Celestial objects are mapped according to a coordinate system on the Celestial Sphere (Fig. 13), 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.
Fig. 13: Celestial Sphere.
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. 13) 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. 13) is a projection of the Earth's Equator onto the celestial 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 latitude (+34°) and longitude (118°); 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°.
All celestial objects are specified in position by their celestial coordinates of Right Ascension and Declination.
- 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 ("hr"), minutes ("min") and seconds ("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, "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".
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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).
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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.
- Make sure the viewfinder is aligned with the ETX-90EC (see Aligning the Viewfinder).
- 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.
- 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.
- 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.
- 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
- 32.5° to 48.5° uses High-Latitude hole (2, Fig. 15).
- 22° to 35.5° uses Alternate hole (3, Fig. 15).
High-Latitude Tripod Leg
- 56° to 66° uses High-Latitude hole.
- 44° to 55° uses Alternate hole.
Thread the appropriate leg into the required hole to a firm feel only.
- A small thumbscrew (6, Fig. 15) is attached to both the standard and high-latitude tripod legs. Loosening the thumbscrew allows the outer section of the leg to slide over the inner section, so that the length of the leg can be extended. If using the standard tripod leg, extend the leg so that the center of the thumbscrew-head is lined up with the latitude of the observing location on the scale. Then retighten the thumbscrew to a firm feel. (If using the high-latitude tripod leg, final adjustment of the leg extension is completed in step 9.)
Example: The latitude of New York City is 41°. The tripod leg should be extended so that the center of the thumbscrew is set next to the 41° reading on the scale.
CAUTION: Polar alignment at latitudes between 22° and 30° requires that the optional #1422 Low-Latitude Balance Weight (8, Fig. 15) be attached to the adjustable leg to stabilize the ETX-90EC for observing.
NOTE: With the standard tripod leg threaded into the appropriate hole in the drive base, the latitude scale may be at an inconvenient position for reading (e.g., the scale may be facing the drive base). This situation can be remedied by unthreading the leg, removing the thumbscrew, rotating the inner leg 180°, then reinserting the thumbscrew. The scale should now be readable when threaded back into the telescope base.
- Loosen the vertical and horizontal locks (6 and 10, Fig. 1) and rotate the telescope so that it is oriented as shown in Fig. 15. Tighten the vertical and horizontal locks. In this orientation the telescope's optical tube is lined up parallel to the tripod's adjustable leg.
- Note the dotted line and arrow extending from the telescope tube in Fig. 15. This line defines the telescope's polar axis. Lift the entire telescope, including tripod, and place the telescope on a firm and level surface so that this axis is pointing due North. For example, if the location of Polaris, the North Star, is known then point the telescope directly at Polaris.
- If using the high-latitude tripod leg in the northern hemisphere, extend the leg until the telescope's polar axis points to Polaris, or due North, an alignment obtained by sighting along the telescope tube with the telescope oriented as shown in Fig. 15.
NOTE: Observer's located in the earth's southern hemisphere (e.g., South America, Australia, etc.) should point the telescope's polar axis due South.
Fig. 15: Polar Alignment using the #880 Table Tripod. (1) Standard Tripod Leg with Latitude Scale; (2) High Latitude Hole; (3) Alternate Hole; (4) Fixed Tripod Legs; (5) Declination Pointer; (6) Thumbscrew; (7) R.A. Scale Pointer; (8) optional #1422 Low-Latitude Balance Weight.
- With the telescope now polar-aligned the table tripod should not be moved, or else polar alignment will be lost. Motions of the telescope (e.g., to locate and/or track objects) should be effected only (a) by loosening the locks (6 and 10, Fig. 1), which permits the optical tube to be moved freely within the telescope mounting, or (b) more generally, with the locks in their "locked" positions, by using the arrow keys of the Electronic Controller.
Important Note: For almost all astronomical observing requirements approximate settings of the telescope's latitude and polar axis are acceptable! Do not allow undue attention to precise polar alignment of the telescope to interfere with your basic enjoyment of the instrument.
In those unusual cases where more precise polar alignment is desirable, refer to APPENDIX C.
Fig. 16: Locating Polaris.
Chapter 4: Observing