Field of View in a Telescope

Mike Swanson

After spending just a little time exchanging the eyepieces in a telescope, you will soon find that more than just the magnification varies with each eyepiece. One of the primary differences is the total amount of sky you can see with each eyepiece, also known as the field of view or FOV.

The field of view is the circle of sky visible through the eyepiece. Generally speaking, as you exchange eyepieces to get a higher magnification, the field of view is a smaller piece of the sky. We measure FOV in degrees or fractions of a degree. Usually astronomers refer to the actual field visible in the eyepiece as the true field of view or TFOV. Knowing the TFOV of each of our eyepieces is very useful since we can then compare what we see in the eyepiece to printed or computerized star charts to help us identify what we are seeing.

Also, some objects require a wide field of view to show the entire object, so we need to choose the eyepiece that will let us 'take it all in'. Herein lies the popularity of wide field eyepieces. At any given magnification, they allow us to see more of the sky. Extended objects like open star clusters, many nebula and some nearby galaxies are only visible in their entirety with a wide-field view.

Calculating the TFOV is not too hard, but there are a few things we need to proceed. First, we need to know the focal length of our telescope and the subject eyepiece. These two are easy as they are generally marked on the side of the scope and eyepiece. But we also need to know the apparent field of view (AFOV) of the eyepiece. This is generally obtainable from the manufacturer of the eyepiece, but it is useful to know that most Plossl eyepieces (the most common type on the market) have an AFOV of 50 degrees.

Armed with this information, the calculations are quite simple. First calculate the magnification of the eyepiece:

MAG = Scope Focal Length / Eyepiece Focal Length

Then you can directly calculate the true field of view:

TFOV = Eyepiece AFOV / MAG

Let's say the focal length of our scope is 1000mm and the focal length of our Plossl eyepiece (50 degrees AFOV) is 10mm:

MAG = 1000 / 10
MAG = 100x

TFOV = 50 / 100
TFOV = .5 degrees

To help with all this math, there is a "Scope Calculator" available in the Downloads section of our Club web site. This Excel spreadsheet performs all the tedious work, you just provide information such as the focal lengths of your scope and eyepieces and the apparent field of view of your eyepieces. The result is a table showing the magnification and the approximate true field of view of each eyepiece. Print a copy and place it in your eyepiece case for quick reference.

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