Calculating Magnification (Power); Determining The Field - Celestron C4.5 Instruction Manual

Calculating
Magnification
Determining Field of
View
|24 © Getting Started
You can change the power of your Celestron C4.5 just by changing the
eyepiece (ocular). To determine the magnification for your C4.5, simply
divide the focal length of the telescope by the focal length of the eyepiece you
are using. 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 a 12.5mm eyepiece. To determine
the magnification, simply divide the focal length of your C4.5 (900mm) by
the focal length of the eyepiece (12.5mm). This yields a magnification of 72
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 C4.5 is 4.5" in
diameter. Multiplying 4.5 by 60 gives a maximum useful magnification of
270 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 90 and 135 times for the C4.5.
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 first calculate
the magnification. Using the example we started with above, we can deter-
mine the field of view using the same 12.5mm eyepiece. The 12.5mm Ultima
eyepiece has an apparent field of view of 51°. Divide the 51° by the magnifi-
cation, which is 72 power. This yields an actual field of 0.7°, or almost three-
quarters of a degree.
This formula gives you the true field of view in degrees. To convert degrees
to feet at 1,000 yards, which is more commonly used for terrestrial viewing,
simply multiply by 52.5. Continuing with our example, multiply the angular
field of .7° by 52.5 produces a linear field width of 36.7 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).