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Magnification =
Let's say, for example, you are using the 10mm eyepiece that came with your telescope. To determine the
magnification you simply divide the focal length of your telescope (the SkyScout Scope for this example has a focal
length of 660mm) by the focal length of the eyepiece, 10mm. Dividing 660 by 10 yields a magnification of 66
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 SkyScout Scope is 3.5
inches (90mm) in diameter. Multiplying 3.5 by 60 gives a maximum useful magnification of over 212 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 70 to 123 times for the SkyScout Scope. You can determine the magnification for your
telescope the same way.
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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 10mm eyepiece that is supplied standard
with the SkyScout Scope. The 10mm eyepiece has an apparent field of view of 50°. Divide the 50° by the
magnification, which is 66 power. This yields an actual field of .75°.
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 of .75° by 52.5. This produces a linear field width of 39
feet at a distance of one thousand yards.
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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.
•
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.
⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯
Focal Length of Eyepiece (mm)
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Apparent Field of Eyepiece
Magnification
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