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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 Angular 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 20 mm eyepiece that is supplied standard
with the Travel Scope. The 20 mm eyepiece has an apparent field of view of 50°. Divide the 50° by the
magnification, which is 20 power. This yields an actual (true) field of 2.5°.
To convert degrees to feet at 1,000 yards (which is more useful for terrestrial observing) multiply by 52.5. Multiply
the angular field of 2.5° by 52.5. This produces a linear field width of 131 feet at a distance of one thousand yards.
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When using 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.
Note: Your telescope was designed for terrestrial observation. Knowing how to use it for this purpose has been
described already as it is quite simple and straightforward. Your telescope can also be used for casual
astronomical observing which will be discussed in the next sections.
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