8
Tracking Celestial Objects
When you observe a celestial object through the telescope,
you’ll see it drift slowly across the field of view. To keep it
in the field, if your equatorial mount is polar aligned, just
turn the R.A. slow-motion control knob clockwise. The
Dec. slow-motion control knob is not needed for tracking.
Objects will appear to move faster at higher magnifica-
tions, because the field of view is narrower.
Optional Motor Drives for Automatic Tracking
An optional DC motor drive can be mounted on the R.A.
axis of the equatorial mount to provide hands-free track-
ing. Objects will then remain stationary in the field of view
without any manual adjustment of the right ascension
slow-motion control knob.
Understanding the Setting Circles
The setting circles on an equatorial mount enable you to
locate celestial objects by their “celestial coordinates”.
Every object resides in a specific location on the “celes-
tial sphere”. That location is denoted by two numbers: its
right ascension (R.A.) and declination (Dec.). In the same
way, every location on Earth can be described by its lon-
gitude and latitude. Right ascension is similar to longitude
on Earth, and declination is similar to latitude. The R.A. and
Dec. values for celestial objects can be found in any star
atlas or star catalog.
The R.A. setting circle is scaled in hours, from 1 through 24,
with small marks in between representing 10-minute incre-
ments (there are 60 minutes in 1 hour of right ascension).
The lower set of numbers apply to viewing in the Northern
Hemisphere, while the numbers above them apply to view-
ing in the Southern Hemisphere. The location of the right
ascension coordinate indicator arrow is shown in Figure 9.
The Dec. setting circle is scaled in degrees, with each mark
representing 2° increments. Values of declination coordi-
nates range from +90° to -90°. The 0° mark indicates the
celestial equator. When the telescope is pointed north of
the celestial equator, values of the declination setting circle
are positive; when the telescope is pointed south of the
celestial equator, values of the declination setting circle are
negative.
So, the coordinates for the Orion Nebula listed in a star
atlas will look like this:
R.A. 5h 35.4m Dec. –5° 27'
That’s 5 hours and 35.4 minutes in right ascension, and -
5 degrees and 27 arc-minutes in declination (there are 60
arc-minutes in 1 degree of declination).
Before you can use the setting circles to locate objects, the
mount must be accurately polar aligned, and the setting
circles must be calibrated.
Calibrating the Declination Setting Circle
1. Loosen the Dec. lock lever and position the telescope
as accurately as possible in declination so it is parallel
to the R.A. axis, as shown in Figure 4. Re-tighten the
lock lever.
2. Loosen one of the thumbscrews on the Dec. setting
circle, this will allow the setting circle to rotate freely.
Rotate the Dec. setting circle until the pointer reads
exactly 90°. Re-tighten the setting circle thumbscrew.
Calibrating the Right Ascension Setting Circle
1. Identify a bright star in the sky near the celestial equa-
tor (declination = 0°) and look up its coordinates in a
star atlas.
2. Loosen the R.A. and Dec. lock levers on the equatorial
mount, so the telescope optical tube can move freely.
3. Point the telescope at the bright star whose coordinates
you know. Lock the R.A. and Dec. lock levers.
4. Loosen one of the R.A. setting circle thumbscrews (see
Figure 9; this will allow the setting circle to rotate free-
ly. Rotate the setting circle until the R.A. pointer arrow
indicates the R.A. coordinate listed in the star atlas for
the object. Re-tighten the setting circle thumbscrew.
Finding Objects With the Setting Circles
Now that both setting circles are calibrated, look up in a
star atlas the coordinates of an object you wish to view.
1. Loosen the Dec. lock lever and rotate the telescope
until the declination value from the star atlas matches
the reading on the Dec. setting circle. Remember that
values of the Dec. setting circle are positive when the
telescope is pointing north of the celestial equator (Dec.
= 0°), and negative when the telescope is pointing south
of the celestial equator. Retighten the lock lever.
2. Loosen the R.A. lock lever and rotate the telescope until
the right ascension value from the star atlas matches
the reading on the R.A. setting circle. Remember to
use the lower set of numbers on the R.A. setting circle.
Retighten the lock lever.
Most setting circles are not accurate enough to put an
object dead-center in the telescope’s eyepiece, but they
should place the object somewhere within the field of
Figure 9. The R.A. and Dec. setting circles.
Dec. setting
circle
Dec. setting
circle
thumbscrew
(2)
R.A.
indicator
arrow
R.A.
setting circle
thumbscrew
(2)
R.A. setting
circle
Dec.
indicator
arrows
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