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Figure 5-3
The emblem on the end of
the focus knob shows the
correct rotational direction
for focusing the NexStar.
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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 25) to go from
close focus (approximately 20 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 eyepiece. To determine the magnification you simply divide the focal length of your
telescope (the NexStar has a focal length of 1300mm) by the focal length of the eyepiece, 25mm. Dividing 1300 by 25 yields a
magnification of 52 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 4" in diameter. Multiplying 4 by 60 gives a maximum useful
magnification of 240 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 80 to 140 times for the NexStar 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 eyepiece has an apparent field of view of 52°. Divide the 52° by
the magnification, which is 52 power. This yields an actual field of view of 1°.
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 1° by 52.5. This produces a linear field width of 57.8 feet at a distance of one thousand yards. The
apparent field of each eyepiece that Celestron manufactures is found in the Celestron Accessory Catalog (#93685).
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