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The NexStar's focusing mechanism controls the primary mirror which is mounted
on a ring that slides back and forth on the primary baffle tube. The focusing knob,
which moves the primary mirror, is on the rear cell of the telescope just below the
star diagonal and eyepiece. Turn the focusing knob until the image is sharp. If the
knob will not turn, it has reached the end of its travel on the focusing mechanism.
Turn the knob in the opposite direction until the image is sharp. Once an image is in
focus, turn the knob clockwise to focus on a closer object and counterclockwise for
a more distant object. A single turn of the focusing knob moves the primary mirror
only slightly. Therefore, it will take many turns (about 30) to go from close focus
(approximately 25 feet) to infinity.
For astronomical viewing, out of focus star images are very diffuse, making them
difficult to see. If you turn the focus knob too quickly, you can go right through focus
without seeing the image. To avoid this problem, your first astronomical target
should be a bright object (like the Moon or a planet) so that the image is visible even
when out of focus. Critical focusing is best accomplished when the focusing knob is
turned in such a manner that the mirror moves against the pull of gravity. In doing
so, any mirror shift is minimized. For astronomical observing, both visually and
photographically, this is done by turning the focus knob counterclockwise.
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You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification
of your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In
equation format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification =
Focal Length of Eyepiece (mm)
Let’s say, for example, you are using the 25mm Plossl eyepiece. To determine the magnification you simply divide
the focal length of your telescope (the NexStar 8i has a focal length of about 2000mm) by the focal length of the
eyepiece, 25mm. Dividing 2000 by 25 yields a magnification of 80 power.
Although the power is variable, each instrument under average skies has a limit to the highest useful magnification.
The general rule is that 60 power can be used for every inch of aperture. For example, the NexStar is 8" in diameter.
Multiplying 8 by 60 gives a maximum useful magnification of 480 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 160 to 280
times for the NexStar 8 telescope.
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Determining the field of view is important if you want to get an idea of the angular size of the object 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 in the
previous section, we can determine the field of view using the same 25mm eyepiece. The 25mm Plossl eyepiece
has an apparent field of view of 50°. Divide the 50° by the magnification, which is 80 power. This yields an actual
field of .63°, or a little more than a half of a degree.
To convert degrees to feet at 1,000 yards, which is more useful for terrestrial observing, simply multiply by 52.5.
Continuing with our example, multiply the angular field .63° by 52.5. This produces a linear field width of 33 feet at a
Figure 3-3
The emblem on the end of
the focus knob shows the
correct rotational direction
for focusing the NexStar
.