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You can change the power of your telescope just by changing the eyepiece (ocular). To determine the magnification of
your telescope, simply divide the focal length of the telescope by the focal length of the eyepiece used. In equation
format, the formula looks like this:
Focal Length of Telescope (mm)
Magnification =
Focal Length of Eyepiece (mm)
Let’s say, for example, you are using the 25mm eyepiece. To determine the magnification you simply divide the focal
length of your telescope (for example, the NexStar 127 has a focal length of 1500mm) by the focal length of the eyepiece,
25mm. Dividing 1500 by 25 yields a magnification of 60 power.
Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The
general rule is that 60 power can be used for every inch of aperture. For example, the NexStar 127 is 5” (127mm) in
diameter. Multiplying 5 by 60 gives a maximum useful magnification of 300 power. Although this is the maximum
useful magnification, most observing is done in the range of 20 to 35 power for every inch of aperture which is 100 to 175
times for the NexStar 127 telescope.
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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 25mm eyepiece. The 25mm eyepiece has an apparent
field of view of 50°. Divide the 50° by the magnification, which is 60 power. This yields an actual field of view of .83°.
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 .83° by 52.5. This produces a linear field width of 44 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).
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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. 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.
