Meade ETX-90EC 24Appendix C PRECISE POLARz ALIGNMENT SETTING CIRCLES, Setting Circles

24Appendix C: PRECISE POLARz ALIGNMENT; SETTING CIRCLES

APPENDIX Precise Polar Alignment

C 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 enjoyment of the instrument.

If desired, you may obtain more precise polar alignment by first accomplishing basic polar alignment as detailed in Polar Alignment Procedure, page 12, then returning to this procedure:

NOTE:This procedure moves the telescope physically to line up precisely 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. 16).

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 SP26mm 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 both ETX models, 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 it locks in a 90° position to facilitate Alt/Az viewing (see OPTIONALACCESSORIES, page 17).

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 Souther n Hemisphere.

•Declination Setting Circle: The Dec. setting circle (Fig. 37) has been factory set to read the correct Declination of sky objects.

Fig.37: Section of Declination setting circle.

Because 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 aligns with the Dec. pointer (5,Fig.16). Retighten the Dec. knob.

To use the setting circles to find astronomical objects, the ETX 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 use the following 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 23), or use a star chart. Center this object in the telescope’s field of view.

Setting Circles

Both ETX models are 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 either ETX model polar aligned, use the Electronic Controller arrow keys (1, Fig. 6) to move the telescope in R.A. (left and right keys) and Dec. (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 Dec. numbers.

•Right Ascension Setting Circle: Since celestial objects move in R.A., 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.

Fig.36: Section of Right Ascension setting circle.

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. 16).

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 familiar.