A telescope that combines the widest possible field of view with the maximum usable exit pupil. Its magnification is just low enough for the exit pupil to be the same as the observer's maximum dark-adapted pupil size. If the maximum pupil size is taken to be 7.5 mm, the magnification will be approximately 0.13 times the aperture in millimetres. This is usually provided by a telescope of small focal ratio and wide-field eyepiece, and is particularly suited to observing the Milky Way, extended deep-sky objects, and comets. A typical RFT has an aperture of 100–180 mm, a focal ratio of f/4, and a field of view of 2–5°.
Subjects: Astronomy and Astrophysics.