Calculating Magnification

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 your telescope by the focal length of the eyepiece used.

Let's say, for example, you are using a 25mm eyepiece. If the telescope has a 2000mm focal length, we divide 2000 by 25 which gives us 80.... so the magnification is 80. Simple!

Although the power is variable, each instrument under average skies has a limit to the highest useful magnification. The general rule is 60 power (magnification) can be used for every inch of aperture. If the telescope has an 8 inch aperture, multiplying 8 by 60 will give us a maximum useful magnification of 480. Although this is the maximum, most observing is done in the range of 20 to 35 power for every inch of aperture.

Determining Field of View

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 (FoV) divide the apparent field of the eyepiece by the magnification.

You need the magnification before determining the FoV. Using the example above from calculating magnification, we can determine the field of view using the same 25mm eyepiece. The 25mm eyepiece has an apparent field of view of 52°. Divide 52° by the magnification, which is power 80. This yields an actual field of view of .65°.

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 .65 by 52.5. This produces a linear field width of 34 feet at a distance of 1,000 yards.


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