field of view

The field of view (field of vision, or just field, or FOV) of an astronomical observation is the region of the celestial sphere being observed at an instant. The term is also used for telescopes (its field of view), meaning the shape of such observations through it (sized to match the FOV of an actual observation). The actual FOV generally depends upon the instrument in use and that of a particular instrument is often a square within the telescope's circular FOV and may be much smaller; for example, JWST's instruments' FOVs are each on a different small part of the theoretical FOV provided by JWST's mirrors. The term field of view is also used for a quantification of such regions (often what is meant by a particular telescope's FOV) consisting of the region's angular area, aka solid angle, often given in square degrees or square arcminutes, but sometimes in degrees or arcminutes, which can mean a side, a diameter, or some such dimension giving a notion of the size. Example telescope FOVs:

• HST ACS using WFC (wide field channel): 11 square arcminutes.
• same, utilizing its HRC (High Resolution Channel): 750 square arcseconds.
• Hale Telescope: a square with 18.5 arcminute sides.
• Palomar 48 Inch Telescope: 47 square degrees.
• Rubin Observatory: 9.6 square degrees.
• Pan-STARRS PS1: 7 square degrees.
• JWST: varies by instrument over a range of 5 to 10 square arcminutes.

The term field of view is also occasionally used to refer to the range of surveys, similarly indicating the portion of the sky it covers. The term wide field is commonly used to indicate a relatively large FOV ("relatively large" given the particular instrument type and/or purpose).