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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:
Magnification =
Let's say, for example, you are using the 40mm Plossl eyepiece. To determine the magnification you simply divide the focal length of your
telescope (the CPC 8 for example has a focal length of 2032mm) by the focal length of the eyepiece, 40mm. Dividing 2032 by 40 yields a
magnification of 51 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 CPC 8 is 8 inches 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 CPC 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:
True Field =
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 40mm eyepiece. The 40mm Plossl eyepiece has an apparent field of view of 46°. Divide the
46° by the magnification, which is 51 power. This yields an actual field of .9°, or almost a full 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 .9° by 52.5. This produces a linear field width of 47 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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When working with any optical instrument, there are a few things to remember to ensure you get the best possible image.
Never look through window glass. Glass found in household windows is optically imperfect, and as a result, may vary in thickness from
one part of a window to the next. This inconsistency can and will affect the ability to focus your telescope. In most cases you will not
be able to achieve a truly sharp image, while in some cases, you may actually see a double image.
Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot summer days or building
rooftops.
Hazy skies, fog, and mist can also make it difficult to focus when viewing terrestrially. The amount of detail seen under these conditions
is greatly reduced. Also, when photographing under these conditions, the processed film may come out a little grainier than normal with
lower contrast and underexposed.
If you wear corrective lenses (specifically glasses), you may want to remove them when observing with an eyepiece attached to the
telescope. When using a camera, however, you should always wear corrective lenses to ensure the sharpest possible focus. If you have
astigmatism, corrective lenses must be worn at all times.
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Focal Length of Telescope (mm)
Focal Length of Eyepiece (mm)
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Apparent Field of Eyepiece
Magnification
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Figure 5-3
The emblem on the end of
the focus knob shows the
correct rotational direction
CPC.
for focusing the
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