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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 20mm eyepiece that is supplied standard
with the AstroMaster 90 EQ telescope. The 20mm eyepiece has an apparent field of view of 50°. Divide the 50° by
the magnification, which is 50 power. This yields an actual field of 1.0°.
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 1.0° by 52.5. This produces a linear field width of 53
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.
