Celestron Ultima 80 - Manual (Page 12)

Apparent Field of Eyepiece
True Field = 
Magnification
As you can see, before determining the field of view, you must calculate the magnification. Using the example in the
previous section, we can determine the field of view using the same 20mm eyepiece. The 20mm eyepiece has an apparent
field of view of 45°. Divide the 45° by the magnification, which is 45 power. This yields an actual field of 1°, or a full
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 1° by 52.5. This produces a linear field width of over 52.5 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).
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