Celestar Manual
Telescope Basics
27
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:
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 25mm eyepiece. The
25mm SMA eyepiece has an apparent field of view of 52°. Divide the 52° by the magnification, which is
81 power. This yields an actual field of .64°, or a little under two-thirds of a degree. The 26mm Pl
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eyepiece has an apparent field of view of 50°. This yields an actual field of .62°.
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 .64° by 52.5. This produces a linear field
width of 33.6 feet at a distance of one thousand yards.
The apparent field of each eyepiece that Celestron manufactures is found in the Celestron Accessory
Catalog (#93685).
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.
•
When using your telescope as a telephoto lens, the split screen or microprism focuser of the 35mm
SLR camera may “black out.” This is common with all long focal length lenses. If this happens, use
the ground glass portion of your focusing screen. To achieve a very sharp focus you may consider
using a focusing magnifier. (These are available from your local camera store.)
•
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.
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