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end of the finder inward or outward until images appear sharp.
Carefully re-tighten the focus lock set-screws. Once the polar
axis finder's objective lens is focused, it should not need to be
adjusted again.
Use of the R.A. and Dec.
Slow-Motion Control Cables
The R.A. and Dec. slow-motion control cables allow fine adjust-
ment of the telescope's position to center objects within the
field of view. Before you can use the cables, you must manu-
ally "slew" the mount to point the telescope in the vicinity of
the desired target. Do this by loosening the R.A. and Dec. lock
levers and moving the telescope about the mount's R.A. and
Dec. axes. Once the telescope is pointed somewhere close to
the object to be viewed, retighten the mount's R.A. and Dec.
lock levers.
The object should now be visible somewhere in the telescope's
finder scope. If it isn't, use the slow-motion controls to scan the
surrounding area of sky. When the object is visible in the finder
scope, use the slow-motion controls to center it. Now, look in
the telescope's 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 eyepiece, use the slow-motion
controls to center it in the field of view.
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 clockwise. 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 can be mounted on the R.A. axis
of the equatorial mount to provide hands-free tracking. Objects
will then remain stationary in the field of view without any man-
ual adjustment of the R.A. slow-motion control cable.
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 marks in between representing 10-minute incre-
ments (there are 60 minutes in 1 hour of R.A.). The upper
set of numbers apply to viewing in the Northern Hemisphere,
while the numbers below them apply to viewing in the Southern
Hemisphere. The location of the R.A. coordinate indicator
arrow is shown in Figure 3d.
The Dec. setting circle is scaled in degrees, with each mark
representing 2° increments. Values of Dec. coordinates range
from +90° to -90°. The 0° mark indicates the celestial equator.
8
When the telescope is pointed north of the celestial equator,
values of the Dec. setting circle are positive; when the tele-
scope is pointed south of the celestial equator, values of the
Dec. 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 well polar aligned, and the R.A. setting circle
must be calibrated. The Dec. setting circle has been calibrated
at the factory, and should read 90° whenever the telescope
optical tube is parallel with the R.A. axis.
Calibrating the Right Ascension Setting Circle
1.
Identify a bright star in the sky 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 whose coordinates
you know. Lock the R.A. and Dec. lock levers. Center the
star in the telescope's field of view with the slow-motion
control cables.
4.
Loosen the R.A. setting circle lock thumbscrew (see
Figure 3d); this will allow the setting circle to rotate
freely. Rotate the setting circle until the arrow under the
thumbscrew indicates the R.A. coordinate listed in the star
atlas for the object. Do not retighten the thumbscrew when
using the R.A.
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 Dec. 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 R.A. value from the star atlas matches the reading on
the R.A. setting circle. Remember to use the upper 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 view of the finder
scope, assuming the equatorial mount is accurately polar
aligned. Use the slow-motion controls to center the object in
the finder scope, and it should appear in the telescope's field
of view.
The R.A. setting circle must be re-calibrated every time you wish
to locate a new object. Do so by calibrating the setting circle for
the centered object before moving on to the next one.
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