The angular diameter of the region of the sky that is well imaged by the telescope is called the telescope's field of view or angle of view (ω). The field of view of a telescope is often determined at design time. The field of view of a telescope is inversely proportional to the magnification, the larger the magnification, the smaller the field of view. Different optical systems, different imaging qualities (caused by aberrations), different apertures, and different focal lengths determine the size of the different fields of view of the telescope
The field of view of a telescope is generally represented by the observable width at a kilometer, so when we look at the telescope parameters, the field of view of a kilometer is generally represented by, for example, 128M/1000M. The field of view of a telescope is generally expressed in three ways:
1）Directly expressed by the field of view, for example, the actual field of view: 7.3° (Note: the field of view is based on the center of the objective lens as the reference point, and the field of view seen through the unadjusted telescope, The angle formed by this field of view range and the reference point is called the field of view angle. The larger the field of view, the easier it is to find the target, and the larger the magnification, the smaller the field of view.)
2） Expressed by the visible range in kilometers, for example, the field of view: beyond 120M/1000M.
3） Expressed in feet at thousand yards, for example, field of view: beyond 120FT/1000Y.