Calculating
Magnification
Determining Field of View
As implied in the previous section, you can change the power of your Celestron telescope just by changing the eyepiece (ocular). To determine the magnification for your telescope, simply divide the focal length of the telescope (1000mm) 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 take an example to see how this formula works. If you were using the standard 20mm eyepiece supplied with your
Although the power is variable, each instrument 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
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. As you can see, before you determine the field of view, you must first calculate the magnification. In equation format, the formula looks like this:
Apparent Field of Eyepiece
True Field =
Magnification
Using the example we started with above, we can determine the field of view using the same 20mm eyepiece. The 20mm eyepiece has an apparent field of view of approximately 52°. Divide the 52° by the magnification, which is 50 power. This yields an actual field of 1.04°, or a little more than 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, 1.04 times 52.5 produces a field size of 55 feet at 1,000 yards.
The apparent field of each eyepiece that Celestron manufactures is found in the Celestron accessory catalog (#93685).
Telescope Basics • 31