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Telescope Basics • 27
You can change the power of your Celestron CM-1100 telescope just by
changing the eyepiece (ocular). To determine the magnification of your
Celestron CM-1100, 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, that you are using the standard 26mm eyepiece. To
determine the magnification you simply divide the focal length of your Celestron
CM-1100 (2800mm) by the focal length of the eyepiece (26mm). Dividing 2800
by 26 yields a magnification of 108 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 Celestron CM-1100 is
11" in diameter. Multiplying 11 by 60 gives a maximum useful magnification of
660 power. Although this is the maximum useful magnification, most observ-
ing is done in the range of 20 to 35 power for every inch of aperture which is
220 to 385 times for the CM-1100.
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 figure the
magnification. Using the example in the previous section, we can determine
the field of view using the same 26mm eyepiece. The 26mm Plössl eyepiece
has an apparent field of view of 50°. Divide the 50° by the magnification, which
is 108 power. This yields an actual field of .46°, or about one half of a degree.
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 .46° by 52.5. This produces a linear field width of 24.2 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).
Calculating
Magnification
Determining Field of
View