Field of View
By: Sam Pitts 2001
What is my Field of View
This depends on your eyepiece’s magnification and apparent field of view, along with the telescope being used. The eyepiece has a focal length indicated in millimeters. The longer the focal length of an eyepiece (25mm-50mm) the lower the power (magnification) and the wider the field of view. This assumes we are using the same telescope with a fixed focal ratio (f/10) and length (fl/2000). The higher the power or magnification the shorter the focal length in millimeters, resulting in a smaller field of view (FOV).
To determine the magnification of an eyepiece, divide its focal length into the focal length of the telescope’s objective lens or mirror. 8″ f/10= 2000mm focal length (fl).
Telescope: 8″ f/10 – 2000mm fl (focal length)
Plossl Eyepiece 32mm with 50° FOV (apparent field of view)
Magnification 2000mm ÷ 32mm = 62.5x
The field of view with this setup is determined by eyepiece magnification and apparent field of view 50°. Hold an eyepiece and look through it. The circular view of light observed is its apparent field of view. The diameter of this circle is the apparent field of view measured in degrees. Below is a list of apparent field of views with different types of eyepieces.
Find the Field of View
To find the actual field of view, divide the eyepiece’s apparent field of view by the magnification on a particular scope. Using the example above (8′ f/10 scope ).
50° ÷ 62.5 = 0.8°
The 32mm Plossl on an 8″ f/10 (2000mm) telescope will render a true field of view of 48 arc minutes or 8/10 of a degree. Remember the moon is approximately 1/2° (30′) in diameter. Wide field of view lenses may suffer from aberration near the edges due to astigmatism. The stars may be slightly distorted near the edge of the field of view.
A 32mm Tele Vue Nagler 82°, with the same telescope, would have a FOV of 1.312° or 1° 18′ 43.2″
A 32mm Kellner 40°, with the same telescope, would have a FOV of .64° or 38′ 24″
Eyepieces > 32mm are best used with 10″ or larger objectives/mirror and a 2″ diagonal.