Editing Optical telescope (section)

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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.