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Meade ETX-90EC Astro Telescope Instruction Manual
 Appendix C: PRECISE POLAR ALIGNMENT; SETTING CIRCLES
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.
[ toc ] Precise Polar Alignment
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.

If desired, more precise polar alignment may be obtained by first accomplishing basic polar alignment as detailed in Polar Alignment Procedure, then returning to this procedure:

NOTE: This procedure moves the telescope physically to precisely line up with the celestial pole. Do not use the Electronic Controller arrow keys to move the telescope electronically or polar alignment will be lost.
  1. Orient the entire telescope, including tripod or tripod legs, so that the polar axis is pointing toward Polaris (Fig. 15).
  2. While observing through the SP 26mm eyepiece of the telescope, adjust the length of the adjustable tripod leg until Polaris is visible in the eyepiece. Use a combination of (a) lifting and turning the entire telescope (or nudging the position of one of the fixed tripod legs) and (b) adjusting the length of the adjustable tripod leg to place Polaris in the center of the telescope's field.
  3. Repeat step 2 of this procedure in about 15 minutes to see how much drift has taken place and to make the alignment more precise.

Although the above procedure is somewhat tedious (since the field of view of the telescope with the SP 26mm eyepiece is only about 1°), it is a worthwhile effort if precise polar alignment is desired (e.g., if photography of the Moon or a planet is to be performed). With Polaris placed in the center of the telescope's eyepiece, the telescope is now polar aligned within about one or two degrees - a level of alignment precision more than sufficient for almost any observing application.

To provide the most stable platform from which to polar align the ETX-90EC it is recommended to purchase the #883 Deluxe Field Tripod. The tripod head tilts easily to the local latitude angle for quick polar alignment, and locks in a 90° position to facilitate Alt/Az viewing (see OPTIONAL ACCESSORIES).

[ toc ] Setting Circles
The ETX-90EC is equipped with R.A. and Dec Setting Circles (14 and 18, Fig. 1) to aid in locating faint celestial objects when the telescope has been polar aligned. Setting circles emulate the celestial coordinates found on star charts or in sky catalogs. Any charted object is easily located by coordinates in R.A. (in hours, minutes, and seconds, from 0h 0m 0s to 23h 59m 59s and Dec (in degrees from 0° to 90°).

With the ETX-90EC polar aligned the Electronic Controller arrow keys (1, Fig. 5) are used to move the telescope in Right Ascension (left and right keys) and Declination (up and down keys).

NOTE: The Dec setting circle is located on the left arm of the telescope fork mount. The right arm of the mount contains a graduated circle (mounted behind the knurled knob of the vertical lock), without Declination numbers.


Fig. 36: Section of Right Ascension setting circle.

  • Right Ascension Setting Circle: Since celestial objects move in Right Ascension the R.A. setting circle (Fig. 36) must be reset as each object is located during an observing session. The R.A. pointer is located on the drive base 90° counterclockwise from the telescope's computer control panel (11, Fig. 1) immediately under the R.A. circle.

NOTE: The R.A. circle has two rows of numbers from 0 to 23, corresponding to the hours of a 24-hour clock. The upper row of numbers is used by observers in the Earth's northern hemisphere, the lower row by observers in the Earth's southern hemisphere.


Fig. 37: Section of Declination setting circle.

  • Declination Setting Circle: The Dec setting circle (Fig. 37) has been factory set to read the correct Declination of sky objects. Since the smooth knob on this fork mount arm need never be loosened, the Dec setting circle should always remain calibrated.

    If for some reason this knob becomes loose and the Dec setting circle must be recalibrated, level the optical tube (5, Fig. 1) so that it is parallel to the drive base. Loosen the smooth knob covering the Dec setting circle until the setting circle moves freely. Reposition the setting circle so that the "0" setting is lined up with the Dec pointer (5, Fig. 15). Retighten the Dec knob.

To use the setting circles to find astronomical objects, the ETX-90EC must first be polar aligned; it is advisable that the motor drive be turned on (see Modes of Operation, page 11) and that a low-power eyepiece (e.g., the SP 26mm eyepiece) be employed. Then follow this procedure:

  1. Identify the celestial coordinates (R.A. and Dec) of a bright, easy-to-find object, such as a bright star. (Avoid using Polaris or any object near Polaris.) Coordinates of some bright stars are listed in the Star Locator (page 22), or use a star chart. Center this object in the telescope's field of view.
  2. Manually turn the R.A. circle (14, Fig. 1) to read the R.A. of the object at the R.A. pointer (7, Fig. 15).
  3. The R.A. circle is now calibrated to read the correct R.A. of any object at which the telescope is pointed. The Dec circle is already calibrated through polar alignment.
  4. To find another object, again identify the R.A. and Dec coordinates. Then, without touching the setting circles, move the telescope (manually, by unlocking the vertical and horizontal locks, or by slewing the telescope using the Electronic Controller arrow keys) so that the R.A. and Dec pointers read the coordinates of the second object.
  5. If the above procedure has been followed carefully, the second object will now be in the telescope's field of view.

NOTE: Since the second object (i.e., the object to be located) is in constant motion, once the R.A. circle is calibrated (step 2 above) the telescope should be moved rapidly to read the coordinates of the second object. Otherwise the second object will no longer be in the position indicated by the R.A. circle.

Using setting circles requires a developed technique. When using the circles for the first time, try hopping from one bright star (the calibration star) to another bright star of known coordinates. Practice moving the telescope from one easy-to-find object to another. In this way the precision required for accurate object location becomes evident.

Appendix D: Roadmap to the Stars

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