Handleiding Meade ETX-90AT (pagina 54 van 60) (Engels)

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Setting Circles

The ETX models are equipped with R.A. and Dec. setting circles (13 and 16, 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 an ETX model polar aligned, use the Arrow keys 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.

• Declination Setting Circle: The Dec. setting circle (Fig. 50) has been factory set to read

the correct Declination of celestial 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 (the molded triangle beneath the setting circle).

Retighten the Dec. knob.

•

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

circle (Fig. 51) 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 (10, Fig. 1) immediately under the R.A. circle.

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 and that a low-power eyepiece (

e.g., a

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 bright stars

are listed in astronomy magazines, text books or star charts. Center this object in the

telescope’s field of view.

2. Manually turn the R.A. circle (

13, Fig. 1) to read the R.A. of the object at the R.A.

pointer (the molded triangle beneath this setting circle).

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 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.

Polaris

Little Dipper

Big Dipper

Cassiopeia

Fig. 49: Locating Polaris

141312

1112 10

Fig. 50: Section of

Declination setting circle.

Fig. 51: Section of Right

Ascension setting circle.

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