Meade ETX-125PE Instruction Manual download pdf (Page 24)

Appendix C: PRECISE POLAR ALIGNMENT; SETTING CIRCLES

APPENDIX

Precise Polar Alignment

I m p o r tant note: For almost all astronomical observ i n g

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).

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.

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 Eart h ’s Nort h e rn

hemisphere, the lower row by observers in the Earth’s Southern

Hemisphere.

• Declination Setting Circle: The Dec. setting circle (Fig. 37)

has been factory set to read the correct Declination of sky

objects.

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.

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.

Fig.37: Section of Declination setting circle.

Fig.36: Section of Right Ascension setting circle.

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