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DETERMINING FIELD OF VIEW
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 25 mm eyepiece.
The 25 mm eyepiece has an apparent field of view of 40°. Divide the 40° by the magnification, which
is 40 power. This yields an actual field of view of 1.0°.
To convert degrees to feet at 1,000 yards, which is more useful for terrestrial observing, simply mul-
tiply by 52.5. Continuing with our example, multiply the angular field 1.0° by 52.5. This produces a
linear field width of 52.5 feet at a distance of one thousand yards.
GENERAL OBSERVING HINTS
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.
28
Apparent Field of Eyepiece
Magnification
COMPUTERIZED TELESCOPE
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