Field of view
The field of view refers to the portion of the sky that a telescope or optical instrument can see in a single observation. It is measured in degrees or arcminutes and determines which types of objects can be comfortably observed. A wide field of view is ideal for extended objects such as nebulae or star clusters; a narrow field of view is better suited for planets or double stars.
Actual Field of View and Apparent Field of View
It is important to distinguish between two concepts. The apparent field of an eyepiece is the angle of view as perceived when looking through the eyepiece— typically between 40 and 110 degrees, depending on the type of eyepiece. The actual field of view is the actual portion of the sky covered, which depends on both the eyepiece’s apparent field of view and the magnification used. The higher the magnification, the smaller the actual field of view.
Choosing the Right Field of View
The choice of field of view depends on the object being observed. The Pleiades, which span several degrees, require a wide field of view— binoculars or a low-magnification telescope. The planet Jupiter, which measures only a few tens of arcseconds, is best observed at high magnification with a narrow field of view. The Orion Nebula, with an apparent diameter of about 1 degree, falls somewhere in between.
Importance in Astrophotography
In astrophotography, the field of view determines which objects fit within the frame. Modern digital sensors come in various sizes, and their field of view depends on both the sensor size and the telescope’s focal length. Photographers choose their telescope-sensor combination based on the objects they wish to capture.
Practical Example
The Moon has an apparent diameter of about 0.5 degrees. With an eyepiece providing a real field of view of 1 degree, it fits comfortably within the field of view with a little room to spare. With an actual field of view of 0.3 degrees, only a portion of the Moon is visible at a time. This simple example illustrates the importance of choosing the right eyepiece for the object being observed.
Frequently Asked Questions
How do you calculate an eyepiece’s real field of view?
The real field of view is obtained by dividing the eyepiece’s apparent field of view by the magnification used. For example, an eyepiece with an apparent field of view of 60 degrees used at a magnification of 100x yields a real field of view of 0.6 degrees.
Do binoculars have a wide field of view?
Generally, yes. Standard binoculars offer fields of view ranging from 5 to 8 degrees, making them ideal for observing extended objects such as star clusters, constellations, or the Milky Way. This is why they are often recommended for beginners in astronomy.
Is a wide field of view always preferable?
No. A wide field of view often goes hand in hand with lower magnification, which reduces the level of detail visible in the objects. The choice always depends on the object being observed and the purpose of the observation.