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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.
Fig. 37: Section of Declination setting circle.
