Field of View Telescope: Definition, Calculation, Comparison, Settings

Field of view of a telescope is understood as the range of angular directions in which objects can be observed for a fixed orientation. Field of view tells how much of the sky we can see through the eyepiece at a single moment. The size of this visible patch, called the true field of view, is determined by the focal length of the telescope joined with the field-stop diameter of the eyepiece, and it is calculated by dividing the apparent field of view by the resulting magnification. Because magnification itself depends on the focal lengths of both telescope and eyepiece, any change in either part alters the extent of sky presented to the observer, making field of view a direct, predictable outcome of the chosen set-up.

What is the field of view (FOV) of a telescope?

Field of view refers to how much sky we can see, and the true field of view is degrees shown through the eyepiece of the telescope. Larger field of view means more sky, and telescope FOV equals eyepiece FOV divided by magnification.

Field of view (FOV) is the circle of sky visible through the eyepiece; it is the angular extent of the observable world seen at any given moment and is measured in degrees or fractions of a degree. For a telescope, this angular area is understood as the range of angular directions in which objects are observed for a fixed orientation of the instrument. The total amount of sky you can see with each eyepiece is known as the field of view, and when a full moon fills the entire FOV of the telescope the circle is about half a degree across.

The true field of view (TFOV) identifies how many degrees of the sky are covered by the diameter of the circular view observed through the telescope with any particular eyepiece in place. It is the number of degrees your eyepiece shows you when you use it with your telescope. Because the field of view is dependent on the magnification of the telescope-eyepiece combination, the apparent field of view (AFOV) figure printed on the eyepiece body is not the field of view you will actually observe. For example, an Optilix 2” eyepiece marked 60° gives TFOV = 60°/51 = 1.18° when used with a telescope of 51× magnification.

Survey telescopes demonstrate the same principle on a larger scale. The VISTA telescope has a field of view of 0.6 sq. degrees, the 1.8 m (5.9 ft) Pan-STARRS telescope has a field of view of 7 sq. degrees, and the UK Schmidt Telescope reaches 30 sq. degrees, while the human eye without instruments spans about 210 degrees.

How to calculate the field of view of a telescope?

To calculate the field of view of a telescope, follow the steps explained below.

• Use the formula for field of view = (135.3 x D) / LL
• Determine quickest way to calculate TFoV by dividing AFoV of eyepiece by magnification
• Apply formula fov = fovₐ / m (eyepiece FOV / magnification)
• Multiply result by 57.3 to calculate FOV
• Use field stop diameter and focal length to calculate FOV
• Divide eyepiece field stop or AFOV by telescope focal length
• Reference apparent field of view in calculations

If D is measured in millimetres and L is known in millimetres, (135.3 × D)/L yields the field directly in arcminutes.

What is the telescope FOV comparison?

The telescope FOV comparison is given in the table below.

When one telescope has a larger field of view than the other, it means you can see more of the surrounding sky with that telescope. This difference is the total amount of sky you can see with each eyepiece, so the comparison starts by noting whose TFOV is larger. We measure FOV in degrees or fractions of a degree For example, one TFOV = 1.25° while another setup gives TFOV = .5°. If we double the AFOV of the eyepiece, the sky-circle widens, but only if magnification does not rise in step, because TFOV is the apparent field of view divided by magnification. A map set to FOV 1.16 matches the FOV of a telescope with an ASI071 camera, letting you preview exactly how much sky will fit in one frame.

How can you change the field of view in a telescope?

You can change the field of view of a telescope by replacing the eyepiece, because the actual field-of-view is determined by eyepiece properties. A short-focal-length eyepiece yields high magnification and a smaller field, while a long-focal-length eyepiece gives low magnification and a wider field. Adding a Barlow lens increases the effective focal length and narrows the field, whereas a focal reducer shortens the focal length and widens the field without swapping the eyepiece. Optical designs that incorporate an additional field lens expand the visible field for a given magnification, and photographic objectives match the apparent field of view to the human eye.