Table of Contents
Advertisement
8
but distant, land object, such as the top ot a telephone
pole. Center the object precisely in the main telescope s
field and lock the R.A. lock lever and Dec lock lever so that
the telescope does not move away from the object.
c.
While looking through the viewfinder, turn one or more of
the three front viewfinder alignment screws (4), Fig. 8. until
the crosshairs of the viewfinder point at precisely the same
position as the main telescope. Tighten the knurled lock
nut on each of the front alignment screws until the lock nuts
are flush against the bracket ring.
The viewfinder is now aligned with the main telescope. Unless
the alignment screws are disturbed, the viewfinder will remain
aligned indefinitely.
3. Using the Viewfinder
Now, to locate any object, terrestrial or astronomical, first center
the object on the crosshairs of the viewfinder; the object will then
be centered in the field of the main telescope.
Note:
If higher observing magnifications are desired, first
locate, center, and focus the object using a low-power eyepiece
(e.g.. the PL 25mm eyepiece). Objects are easier to locate and
center at low powers: higher powers may then be employed by
changing eyepieces.
MAGNIFICATION
The magnification, or power, at which a telescope is operating is
determined by two factors:
the focal length of the eyepiece
employed and the focal length of the telescope.
The Model
102ACHR/500 telescope is supplied with one eyepiece as
standard equipment. The focal length of the eyepiece, 25mm,
is printed on its side.
Telescope focal length is, roughly speaking, the distance that
light travels inside the telescope before reaching a focus. The
focal length of the Model 102ACHR/500 telescope is 920mm
On a given telescope, different eyepiece focal lengths are used
to achieve different magnifications, from low to high. Optional
eyepieces and the Series 4000 #140 2x Barlow Lens are
available for powers from a range of about 25x to 400x.
depending on the focal length of the telescope and eyepiece
(see OPTIONAL ACCESSORIES, page 11).
To calculate the magnification (or power) obtained with a given
eyepiece, use this formula:
Telescope Focal Lengthy
Power
-
Eyepjece Focal Length
Example: Using the PL 25mm eyepiece supplied with the Model
102ACHR/500 telescope, the power is:
Power
920mm
25mm
= 37x
ASTRONOMICAL OBSERVING
Understanding how and where to locate celestial objects, and
how those objects move across the sky is fundamental to
enjoying the hobby of astronomy. This section provides a basic
introduction to the terminology associated with astronomy, and
includes instructions for finding and tracking celestial objects.
Celestial Coordinates:
Declination and Right Ascension
Celestial objects are mapped according to a coordinate system
on the Celestial Sphere, the imaginary sphere on which all stars
appear to be placed. This celestial object mapping system is
analogous to the Earth-based coordinate system of latitude and
longitude.
The poles of the celestial coordinate system are defined as
those two points where the Earth's rotational axis, if extended to
infinity, north and south, intersect the celestial sphere. Thus, the
North Celestial Pole is that point in the sky where an extension
of the Earth's axis through the North Pole intersects the celestial
sphere.
This point in the sky is located near the North Star,
Polaris.
The faint star Sigma Octans lies near the South
Celestial Pole, where the extension of Earth's axis through the
South Pole intersects the celestial sphere.
In mapping the surface of the Earth, lines of longitude are drawn
between the North and South Poles. Similarly, lines of latitude
are drawn in an east-west direction, parallel to the Earth's
Equator. The Celestial Equator is a projection of the Earth's
Equator onto the celestial sphere.
In mapping the celestial sphere, just as on the surface of the
Earth, imaginary lines have been drawn to form a coordinate
grid. Thus, object positions on the Earth's surface are specified
by their latitude and longitude. For example, you could locate
Los Angeles, California, by its latitude (34°) and longitude
(118°); similarly, you could locate the constellation Ursa Major
by its position on the celestial sphere:
The most common mistake of the beginning observer is to
"overpower" the telescope and use high magnifications which
the telescope's aperture and typical atmospheric conditions
cannot reasonably support.
Keep in mind that a smaller but
bright and well-resolved image is far superior to a larger but dim
and poorly resolved one. Powers above about 200x should be
employed only under the steadiest atmospheric conditions.
Most observers will want to have 3 or 4 eyepieces and perhaps
the Series 4000 #140 2x Barlow Lens to achieve the full range
of reasonable magnifications.
See OPTIONAL
ACCES¬
SORIES, page 11, for further details.
R.A.: 11 hr; Dec:+50°.
The celestial analog to Earth latitude is called Declination, or
"Dec'\ and is measured in degrees, minutes and seconds (e.g..
15" 27' 33").
Declination north of the celestial equator is
indicated with a "+" sign in front of the measurement (e.g.. the
Declination of the North Celestial Pole is +90°), with Declination
south of the celestial equator indicated with a
sign (e g., the
Declination of the South Celestial Pole is -90°) (Fig. 9). Any
point on the celestial equator itself (which, for example, passes
through the constellations Orion, Virgo and Aquarius) is
specified as having a Declination of zero, shown as 0° O' 0".
Advertisement
Table of Contents
