Celestron C5 - Calculating Magnification; Determining Field of View

12 • Getting Started
You can change the power of your Celestron C5 spotting scope just by changing the
eyepiece (ocular). To determine the magnification of your Celestron C5, simply
divide the focal length of the spotting scope by the focal length of the eyepiece
used. In equation format, the formula looks like this:
Focal Length of Spotting Scope (mm)
Magnification = ————————————————
Focal Length of Eyepiece (mm)
Let’s say, for example, that you are using a 26mm eyepiece. To determine the
magnification, you simply divide the focal length of your C5 (1250mm) by the focal
length of the eyepiece (26mm). Dividing 1250 by 26 yields a magnification of 48
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 C5 is 5" 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
between 20 to 35 power for every inch of aperture which is 100 to 175 times for
the C5.
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 (in degrees)
True Field =
Magnification
As you can see, before determining the field of view, you must first calculate
the magnification. Using the example in the previous section, we can deter-
mine the field of view using the same 26mm eyepiece. The 26mm Plossl eyepiece
has an apparent field of view of 50°. Divide the 50° apparent field by the magnifica-
tion, which is 48 power. This yields an actual field of 1.04°, or a little over one
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, multiplying
the angular field 1.2° by 52.5 produces a linear field width of 55 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