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Note : From this point on in your observing session, you
should not make any further adjustments in the azimuth
or the latitude of the mount, nor should you move the tri-
pod. Doing so will undo the polar alignment.The telescope
should be moved only about its R.A. and Dec. axes.
Use of the R.A. and Dec. Slow-Motion Control
Cables
The R.A. and Dec. slow-motion control cables allow fine
adjustment of the telescope's position to center objects with-
in the field of view. Before you can use the cables, you must
manually "slew" the mount to point the telescope in the vicini-
ty of the desired target. Do this by loosening the R.A. and
Dec. lock knobs and moving the telescope about the mount's
R.A. and Dec. axes. Once the telescope is pointed some-
where close to the object to be viewed, retighten the mount's
R.A. and Dec. lock knobs.
The object should now be visible somewhere in the tele-
scope's finder scope. If it isn't, use the slow-motion controls
to scan the surrounding area of sky. When the object is visi-
ble in the finder scope, use the slow-motion controls to center
it. Now, look in the telescope with a long focal length (low
magnification) eyepiece. If the finder scope is properly
aligned, the object should be visible somewhere in the field
of view.
Once the object is visible in the telescope's eyepiece, use the
slow-motion controls to center it in the field of view. You can
now switch to a higher magnification eyepiece, if you wish.
After switching eyepieces, you can use the slow-motion con-
trol cables to re-center the image, if necessary.
The Dec. slow-motion control cable can move the telescope
a maximum of 25°, This is because the Dec. slow-motion
mechanism has a limited range of mechanical travel. (The
R.A. slow-motion mechanism has no limit to its amount of
travel.) If you can no longer rotate the Dec. control cable in a
desired direction, you have reached the end of travel, and the
slow-motion mechanism should be reset. This is done by first
rotating the control cable several turns in the opposite direc-
tion from which it was originally being turned. Then, manually
slew the telescope closer to the object you wish to observe
(remember to first loosen the Dec. lock knob).You should now
be able to use the Dec. slow-motion control cable again to
fine adjust the telescope's position.
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 cable. The Dec. slow-motion
control cable is not needed for tracking. Objects will appear
to move faster at higher magnifications, because the field of
view is narrower.
Optional Motor Drives for Automatic Tracking
An optional DC motor drive (Orion EQ-2M Motor Drive,
#7827) can be mounted on the R.A. axis of the equatorial
mount to provide hands-free tracking. Objects will then
8
remain stationary in the field of view without any manual
adjustment of the R.A. slow-motion control.
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 "celestial 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 longitude and latitude. R.A.
is similar to longitude on Earth, and Dec. 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 hash marks in between representing 10-minute
increments (there are 60 minutes in 1 hour of R.A.). The lower
set of numbers (closest to the plastic R.A. gear cover) apply
to viewing in the Northern Hemisphere, while the numbers
above them apply to viewing in the Southern Hemisphere.
The Dec. setting circle is scaled in degrees, with each hash
mark representing 1° increments. Values of Dec. coordinates
range from +90° to -90°, For Northern Hemisphere
observers, use the numbers on the setting circle that are clos-
est to the eastern horizon. The 0° mark indicates the celestial
equator; values north of the Dec. = 0° mark are positive, while
values south of the Dec. = 0° mark 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 well polar aligned, and the R.A. setting circle
must be calibrated. The Dec. setting circle has been calibrat-
ed at the factory, and should read 90° when the telescope
optical tube is parallel with the R.A. axis.
Calibrating the Right Ascension Setting Circle
1. Identify a bright star near the celestial equator (Dec. = 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 near the celestial
equator whose coordinates you know. Lock the R.A. and
Dec. lock knobs. Center the star in the telescope's field of
view with the slow-motion control cables
4. Loosen the R.A. setting circle lock thumb screw located
just above the R.A. setting circle pointer; this will allow the
setting circle to rotate freely. Rotate the setting circle until
the pointer indicates the R.A. coordinate listed in the star
atlas for the object. Retighten the thumb screw.
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.
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