16
TELESCOPE BASICS
Focusing the Telescope
The
Star Hopper
®
telescope is equipped with a rack and pinion focuser. Focusing is accomplished by
rotating the focuser knobs until a sharp focus is reached. To focus your telescope, begin by putting the
eyepiece in the eyepiece holder of the focuser. When doing astronomical viewing, you’ll find that out of
focus star images are very diffuse and difficult to see. Therefore, choose a bright object, like the moon or
a planet for your first astronomical target. This way, the image will be visible even when out of focus. If
you’re focusing on a star, its image should, as closely as possible, resemble a pinpoint. If you’re focusing
on the moon or a planet, rotate the focuser barrel until the image is sharp.
Calculating Magnification
You can change the viewing power of your
Celestron Star Hopper
®
telescope just by changing the
eyepiece. There are various optional eyepieces shown in the Optional Accessories section of this manual.
To determine the magnification of your telescope, simply divide the focal length of the telescope by the
focal length of the eyepiece being used. In equation format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification
=
––––––
Focal Length of Eyepiece (mm)
To determine the magnification using the standard 25mm eyepiece, simply divide the focal length of your
Star Hopper
®
telescope by the focal length of the eyepiece (25mm). For example, dividing the focal
length of the
Star Hopper
®
8" (1200mm) by 25mm yields a magnification of 48 power. Although the
power is variable, each telescope, used under average skies, has a limit to its highest useful
magnification. The general rule is that 60 power is the maximum that can be used for every inch of
aperture, although seeing condition rarely allow this. For example, the
Star Hopper
®
10
is 10" in
diameter. Multiplying 10 by 60 gives a maximum useful magnification of 600 power. Although this is the
maximum useful magnification, most observing is done in the range of 20 to 35 power for every inch of
aperture, which is 200 to 350 for the
Star Hopper
®
10
.
Determining Field of View
Determining the field of view is important if you want to get an idea of the angular size of the object that
you are observing. To calculate the actual field of view, divide the apparent field of the eyepiece (supplied
by the eyepiece manufacturer) by the magnification. In equation format, the formula looks like this:
Apparent Field of Eyepiece
True Field =
Magnification
As you can see, before determining the field of view, you must calculate the magnification. Using the
example above, we can determine the field of view using the same 25mm eyepiece. The 25mm eyepiece
has an apparent field of view of 52°. Divide the 52° by the magnification, which is 48 power for the
Star
Hopper
®
8
. This yields an actual field of 1.08°, or a little over a degree. The apparent field of each
eyepiece that
Celestron
manufactures is found in the
Celestron
Accessory Catalog (#93685).
