The field of view (FOV) is the size of the circle of the sky you can see through the eyepiece of a telescope.
The Maths
FOV is dependent on the apparent field of view of the eyepiece, and the magnification of your setup. It is calculated by dividing the eyepiece FOV by the magnification of the telescope setup (including barlows or field reducers). The higher magnification the narrower the field of view. It is measured in (arc) degrees or fractions of degrees (arc minutes and arc seconds).
Understanding field of view
But why should you worry about the field of view? Planetary observation and deep-sky imaging require very different fields of view. Planets are tiny and need a lot of magnification hence a narrow FOV. Deep-sky objects are much bigger and require a wider FOV.
If we bring in the magnification formula into the field of view formula, you can see that besides the eyepiece characteristics, the field of view only depends on the effective focal length of the telescope.
For the same eyepiece, telescopes with a shorter focal length, therefore, generate a wider field of view and hence are better for observing deep-sky objects, telescopes with a longer focal length generate a narrower field of view and hence are better for observing solar system objects.
Depending on what you want to observe, you will have to choose the corresponding setup …. or get multiple telescopes Understanding field of view is extremely important when choosing your equipment.
A similar analysis will follow when discussing the f-ratio of a telescope. f/ratio brings the aperture of a telescope into the equation, and hence the brightness and resolution of an image.
Experiencing field of view
The images below were generated using the FOV app from astronomy.tools. Field of view apps are great ways to understand what to expect from a telescope before buying one. All you need to do to have an idea of what to expect is:
- choose a planet or a deep sky object you would want to observe
- choose the telescope you had in mind
- an play around with barlows, field reducers, and eyepieces
Notice how in the 2 images below, using the same telescope, different fields of view can be seen by changing eyepieces and adding or removing barlows. The purple field of view is too narrow to observe the Andromeda galaxy but far too wide to observe Jupiter.
Wide field of view
Andromeda (M31) is a huge object in the night sky. Maybe not when you look up into the night sky with your eyes, but if you can expose long enough for it with a camera to capture the faint edges of the galaxy, it shows to be about 3 times as wide as the Moon. It requires equipment with a wide field of view to be imaged.
Narrow field of view
Compared to the Andromeda galaxy (M31), Jupiter is tiny and needs very different equipment to be observed. You need much narrows field of view.
