Orion Observer 90mm Instruction Manual page 10

Northern hemisphere scale
.
Figure 20 A) The declination setting circle. B) The right ascension
setting circle.
slow-motion control cable counterclockwise to track. 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 Electronic Drive for Automatic Tracking
An optional DC electronic 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 manual
adjustment of the R.A. slow-motion control cable.
Understanding the Setting Circles
The two setting circles (Figure 20) on an equatorial mount
enable you to locate celestial objects by their "celestial coordi-
nates". Every object resides in a specific location on the "celestial
sphere" . That location is denoted by two numbers: its right ascen-
10
Arrow
A
Southern hemisphere scale
Arrow
B
Paper
(white)
Figure 21. Collimate the secondary mirror using the three screws
on the secondary mirror "spider". Place a piece of white paper
inside the optical tube opposite the focuser during collimation.
sion (R.A.) and declination (Dec.). In the same way, every loca-
tion 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 at the base of the setting circle scale apply
to the Southern Hemisphere while the numbers above them
apply to viewing in the Northern Hemisphere.
The Dec. setting circle is denoted in degrees, with each main
mark representing 10° increments (1 = 10°, 2 = 20°, etc.). Values
of Dec. coordinates range from +90° to -90°. The 0° mark indi-
cates the celestial equator. For this mount, the number scale
goes to 90 on either side of 0 – there are no (+) or (-) signs.
When the telescope is pointed north of the celestial equator,
values of Dec. are positive, while when the telescope is pointed
south of the celestial equator, values of Dec. are negative.
For example, the coordinates for the Orion Nebula (M42) are:
R.A. 5 hr 35.4 min, 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 polar aligned, and the setting circles must be calibrated.
The easiest way to calibrate the setting circles is to point the tele-
scope at an identifiable bright star, center it in the eyepiece, then
set the setting circles to the star's published coordinates, which
you can find in a star atlas or astronomical software program, or
perhaps on the internet.
Collimation
adjustment
screw (x3)