TFOV

TFOV, or true field of view, is a measure of the angular extent of the sky visible through an optical instrument, as seen through the eyepiece. It represents the actual angular width, as opposed to the apparent field of view (AFOV) of the eyepiece. TFOV is determined mainly by the AFOV of the eyepiece and the magnification produced by the instrument. The standard approximation is TFOV ≈ AFOV / M, where M is the magnification (M = FL_telescope / FL_eyepiece).

For example, a telescope with a 1000 mm focal length using a 25 mm eyepiece (AFOV about 50°) yields M = 1000/25 = 40, and TFOV ≈ 50°/40 ≈ 1.25°. Using a wider AFOV eyepiece or a shorter focal length telescope increases TFOV.

TFOV can be limited by the eyepiece field stop and by optical vignetting or distortion. The field stop diameter of the eyepiece restricts the maximum TFOV, regardless of AFOV, and some designs introduce distortion near the edge of the field, reducing the usable TFOV. In practice, manufacturers may quote TFOV, but actual observed TFOV can differ slightly due to alignment, telescope design, and seeing conditions.

TFOV is commonly used in astronomy to frame objects, estimate object size on the sky, and plan star hopping. It is usually expressed in degrees or arcminutes.