13
Note on Using High Powers –
Higher powers are used mainly for lunar and sometimes planetary observing where
you can greatly enlarge the image, but remember that the contrast and brightness will be very low due to the high
magnification. Using the 4mm eyepiece together with the 3x Barlow lens gives extremely high power and can be
used on rare occasions – you will achieve the power but the image will be dark with low contrast because you have
magnified it to the maximum possible. For the brightest images with the highest contrast levels, use lower powers.
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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 20mm eyepiece that is supplied standard
with the PowerSeeker 60AZ telescope. The 20mm eyepiece has an apparent field of view of 50°. Divide the 50° by
the magnification, which is 35 power. This yields an actual (true) field of 1.4°.
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 1.4° by 52.5. This produces a linear field width of 74
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.
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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.
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Never look across or over objects that are producing heat waves. This includes asphalt parking lots on hot
summer days or building rooftops.
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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.
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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.
