Orion starBlast 4.5 EQ Instruction Manual page 8

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 mount's R.A. setting circle is scaled in hours, from 1 through
24, with small marks in between representing 10-minute incre-
ments. The numbers closest to the R.A. axis gear apply to view-
ing in the Southern Hemisphere, while the numbers above them
apply to viewing in the Northern Hemisphere.
The Dec. setting circle is scaled in degrees, with each mark rep-
resenting 2.5° increments. Values of Dec. coordinates 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 Dec. setting circle are positive, while when the telescope 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-min-
utes in 1 degree of declination).
Before you can use the setting circles to locate objects, the
mount must be properly polar aligned, and the R.A. setting circle
must be calibrated. The Dec. setting circle has been permanently
calibrated at the factory, and should read 90° whenever the tele-
scope 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 knobs 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 knobs. Center the
star in the telescope's field of view with the slow-motion
control cables.
4. Rotate the setting circle until the metal arrow indicates the
R.A. coordinate listed in the star atlas for the object.
Finding objects with the setting circles
1. Now that both setting circles are calibrated, look up in a
star atlas the coordinates of an object you wish to view.
2. Loosen the R.A. lock knob 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
knob.
8
3. Loosen the Dec. lock knob 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 equa-
tor. Retighten the lock knob.
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 EZ Finder II,
assuming the equatorial mount is accurately polar aligned. Use
the slow-motion controls to center the object in the reflex sight,
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.
confused about Pointing the telescope?
Beginners occasionally experience some confusion about how to
point the telescope overhead or in other directions. In Figure 1a
the telescope is pointed north, as it would be during polar align-
ment. The counterweight shaft is oriented downward. But it will
not look like that when the telescope is pointed in other directions.
Let's say you want to view an object that is directly overhead, at
the zenith. How do you do it?
One thing you DO NOT do is make any adjustment to the latitude
adjustment T-bolt. That will nullify the mount's polar alignment.
Remember, once the mount is polar aligned, the telescope should
be moved only on the R.A. and Dec. axes. To point the scope
overhead, first loosen the R.A. lock knob and rotate the telescope
on the R.A. axis until the counterweight shaft is horizontal (paral-
lel to the ground). Then loosen the Dec. lock knob and rotate the
telescope until it is pointing straight overhead. The counterweight
shaft is still horizontal. Then retighten both lock knobs.
Similarly, to point the telescope directly south, the counterweight
shaft should again be horizontal. Then you simply rotate the
scope on the Dec. axis until it points in the south direction.
What if you need to aim the telescope directly north, but at an
object that is nearer to the horizon than Polaris? You can't do it
with the counterweight down as pictured in Figure 1a. Again, you
have to rotate the scope in R.A. so the counterweight shaft is
positioned horizontally. Then rotate the scope in Dec. so it points
to where you want it near the horizon.
To point the telescope to the east or west, or in other direc-
tions, you rotate the telescope on its R.A. and Dec. axes.
Depending on the altitude of the object you want to observe,
the counterweight shaft will be oriented somewhere between
vertical and horizontal.
Figure 8 illustrates how the telescope will look pointed at the
four cardinal directions—north, south, east, and west
The key things to remember when pointing the telescope is
that a) you only move it in R.A. and Dec., not in azimuth or
latitude (altitude), and b) the counterweight and shaft will not
always appear as it does in Figure 1a. In fact, it almost never
will!