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Field of View in Astronomy Explained

The term field of view confuses a lot of people new to the hobby of astronomy. Here’s what to know about the field of view in optical instruments, including apparent vs true and degrees vs arcminutes, to help you demystify this term and get better satisfaction out of using your optical instruments.

field of view in astronomy explained

Define field of view

Field of view is the extent or the range of your visual area.” ~ Optics Den

In astronomy, the field of view “is the angular width of the patch of sky you can see through your optical instrument” (Mosley, 2003). In binoculars it’s expressed as degrees and in telescopes, arcminutes.

Field of view in optical instruments for stargazing

As a rule of thumb, the field of view of optical instruments are as follows:

  • Naked eye — 100º
  • Pair of binoculars — 5º–8º
  • Low power telescope — 0.5º or 30 arcminutes (‘)
  • High power telescope — 0.66º or 10′

Apparent field of view vs True field of view

When it comes to field of view, there’s mention of ‘apparent’ and ‘true’. You might be wondering how these differ. Here are the definitions of true vs apparent field of view:

What is true field of view? The true field of view (TFOV) is what you see of the sky when looking through an optical instrument, either a pair of binoculars, a telescope, or the naked eye.

What is the apparent field of view? The apparent field of view (AFOV) is a fixed property of a telescope eyepiece expressed in degrees. It is the angular diameter of the circle determined by the field stop that’s seen through the eyepiece. It works with the telescope’s magnification to give you the TFOV.

best eyepiece, field of view
Diagram showing the field of view. Source: Randy Culp

field of view in telescope

What is the field of view in a telescope? The field of view in a telescope is the circle of what you see of space when looking through this optical instrument. It’s determined by the telescope’s power and the apparent field of view of the eyepiece. It is typically expressed as arcminutes.

How to calculate TFoV of a telescope – Method 1:

  • True FoV = Apparent FoV ÷ magnification;
    • Where magnification is the telescope focal length ÷ eyepiece focal length. The steps:
      • TFL÷EFL… where the telescope focal length, say 900mm, is divided by the eyepiece focal length, say 20 mm, to give magnification, 45X.
      • AFoV÷magnification… where the AFoV, say 100°, is divided by the magnification, 45X.
      • TFoV is 100/45, giving result of ~2.22° (or 133.3 arcminutes).

The size of the field stop determines the AFoV of the eyepiece. And so, there’s another formula to calculate TFoV for a telescope, based on the field stop dimensions…

Field of View (FOV) Calculation for Telescope – Method 2

TFOV = eyepiece field stop diameter ÷ telescope’s focal length x 57.3

Few brands list this field stop dimension, apart from TeleVue.

Why care about AFoV of eyepieces?

Why does anyone care about the apparent field of view? Isn’t the true field what really matters? Let’s compare eyepieces with the same TFoV and then with the same AFoV to understand how AFoV matters.

same TFoV

Imagine you have a normal drinking straw, 5 mm in diameter and about 200 mm in length and you’re looking through it. The AFoV equals the TFoV, i.e., 1.4325º (5mm x 57.3 ÷ 200mm — see Method 2), noting that the magnification is 1X. You’ll also get this TFoV with a 50º (AFoV) Plossl at 35X (magnification), a 70º superwide at 49X, and an 82º ultrawide at 57X (Method 1).

All the above have the same TFoV, but with different AFoVs you get enormously different experiences across them. Compare what you’d see through a drinking straw (at 1x magnification) to the expanse you’d get with 57X magnification.

same AFoV

Imagine you have two eyepieces with the same AFOV, say 100° but with different focal lengths, say 13 mm and 21 mm (as in TeleVue Ethos eyepieces).

Tele Vue 21mm Ethos 2" Eyepiece
TeleVue 21mm Ethos

available at Amazon

Tele Vue 13mm Ethos 2" / 1.25" Eyepiece with 100 Degree Field of View.
TeleVue 13mm Ethos

available at Amazon

The 13 mm gives a view that’s about double the magnification of the 21 mm — The object looks about twice as big through the 13 as opposed to the 21 mm. But because both have a 100° AFoV, the 13 mm with have a smaller TFoV because you are only fitting in half of the image you get with the 21 mm.

Field of view binoculars

In binoculars, the field of view is expressed as degrees or a linear measure of width (e.g., feet @ 1000 yds).

With binoculars you can enjoy a wider field of view, say 5–8º, compared to that of a telescope (< 1º). This FOV corresponds to four fingers at arms length held up to the sky (see my article on hand measures).

BINOCULAR field of view compared for lower vs higher magnification

Basically, as you go up in power the field of view narrows. This is something to weigh up when choosing astronomy binoculars.

Bottom line

I hope the above graphics and content has removed the confusion and shed light on the meaning of apparent field of view property for eyepieces, and then the true field of view in terms of the naked eye, binoculars, and telescopes.

It’s important to understand these factors as it helps in choosing the right eyepiece and getting the most satisfaction out of using your telescope to observe planets or other objects or with using binoculars for astronomy.

References

  1. Mosley, John. Starry Night Companion. Space Holding Corp. Canada.