Editing Optical telescope (section)

===Field of view===
Field of view is the extent of the observable world seen at any given moment, through an instrument (e.g., telescope or [[binoculars]]), or by naked eye. There are various expressions of field of view, being a specification of an [[eyepiece]] or a characteristic determined from an [[eyepiece]] and telescope combination. A physical limit derives from the combination where the FOV cannot be viewed larger than a defined maximum, due to [[diffraction]] of the optics.

====Apparent====
Apparent field of view (commonly referred to as AFOV) is the perceived angular size of the field stop of the [[eyepiece]], typically measured in [[Degree (angle)|degrees]]. It is a fixed property of the eyepiece's optical design, with common commercially available eyepieces offering a range of apparent fields from 40° to 120°. The apparent field of view of an eyepiece is limited by a combination of the eyepiece's field stop diameter, and focal length, and is independent of magnification used.

In an eyepiece with a very wide apparent field of view, the observer may perceive that the view through the telescope stretches out to their [[peripheral vision]], giving a sensation that they are no longer looking through an eyepiece, or that they are closer to the subject of interest than they really are. In contrast, an eyepiece with a narrow apparent field of view may give the sensation of looking through a tunnel or small porthole window, with the black field stop of the eyepiece occupying most of the observer's vision.

A wider apparent field of view permits the observer to see more of the subject of interest (that is, a wider true field of view) without reducing magnification to do so. However, the relationship between true field of view, apparent field of view, and magnification is not direct, due to increasing distortion characteristics that correlate with wider apparent fields of view. Instead, both true field of view and apparent field of view are consequences of the eyepiece's field stop diameter.

Apparent field of view differs from true field of view in so far as true field of view varies with magnification, whereas apparent field of view does not. The wider field stop of a wide angle eyepiece permits the viewing of a wider section of the [[real image]] formed at the telescope's focal plane, thus impacting the calculated true field of view.

An eyepiece's apparent field of view can influence total view brightness as perceived by the eye, since the apparent angular size of the field stop will determine how much of the observer's retina is illuminated by the [[exit pupil]] formed by the eyepiece. However, apparent field of view has no impact on the apparent [[surface brightness]] (that is, brightness per unit area) of objects contained within the field of view.

====True====
True FOV is the width of what is actually seen through any given eyepiece / telescope combination.

There are two formulae for calculating true field of view:

# Apparent field of view method given by <math>v_{t} = \frac {v_{a}}{M}</math>,  where <math>v_{t}</math> is the true FOV, <math>v_{a}</math> is the apparent field of view of the eyepiece, and <math>M</math> is the magnification being used.<ref name=":0">{{Cite web|date=2017-11-20|title=Simple Formulas for the Telescope Owner|url=https://skyandtelescope.org/observing/stargazers-corner/simple-formulas-for-the-telescope-owner/|access-date=2022-01-28|website=Sky & Telescope|language=en-US}}</ref><ref name=":1">{{Cite web|title=Determine Your True Field of View - Astronomy Hacks [Book]|url=https://www.oreilly.com/library/view/astronomy-hacks/0596100604/ch04s15.html|access-date=2022-01-28|website=www.oreilly.com|language=en}}</ref>
# Eyepiece field stop method given by <math>v_{t} = \frac {d_f}{f_t} \times 57.3</math>, where <math>v_{t}</math> is the true FOV, <math>d_{f}</math> is the eyepiece field stop diameter in millimeters and <math>f_{t}</math> is the focal length of the telescope in millimeters.<ref name=":0"/><ref name=":1"/>

The eyepiece field stop method is more accurate than the apparent field of view method,<ref name=":1"/> however not all eyepieces have an easily knowable field stop diameter.

====Maximum====
Max FOV is the maximum useful true field of view limited by the optics of the telescope. It is a physical limitation where increases beyond the maximum remain at maximum. Max FOV <math>v_{m}</math> is the barrel size <math>B</math> over the telescope's focal length <math>f</math> converted from [[radian]] to degrees.<ref name="SaharaSkyObservatory"/><ref name="RyukyuAstronomyClub"/>

An example of max FOV using a telescope with a barrel size of 31.75&nbsp;mm (1.25 [[inches]]) and focal length of 1200&nbsp;mm is given by: <math>v_{m} = B \cdot \frac {\frac {180}{\pi}}{f} \approx 31.75 \cdot \frac {57.2958}{1200} \approx 1.52^\circ</math>