How do I find the apparent field of view (aFOV) of a eyepiece using simple math (if possible)? I need the aFOV to give as a value to Stellarium’s custom eyepiece

I found a Q&A about this subject but, I don’t fully understand the math:

On my eyepiece it says “Gskyer® 5mm” Also see photo below.

Photo of my 5 mm eyepiece

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    $\begingroup$ Note that the aFOV depends on the detailed design of the eyepiece; it can't be deduced from magnification or focal length. That's why the linked question advises you to measure the field stop or time how long it takes a star to cross the field; some measurement will be required to get the aFOV. Inexpensive eyepieces will have aFOV of about 40 degrees or less; specialized wide-field eyepieces, usually larger and heavier with larger glass elements, can achieve 80 degrees or more. $\endgroup$
    – antlersoft
    Jan 19, 2022 at 22:27
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    $\begingroup$ @uhoh thank you for welcoming me to Stack Exchange $\endgroup$
    – Donnie
    Jan 20, 2022 at 1:08
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    $\begingroup$ @GrapefruitIsAwsome no that link does not answer my question $\endgroup$
    – Donnie
    Jan 20, 2022 at 1:09
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    $\begingroup$ @uhoh, I've retracted my close vote $\endgroup$ Jan 20, 2022 at 1:32
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    $\begingroup$ That eyepiece is going to be a simple design with an AFoV of around 45° (something between 42° and 52°, but probably towards the lower end). The simplest way to estimate it is to look at the Moon with it, remember what you see, and compare it with a FoV calculator (fill in all the numbers you know: telescope aperture and focal length, and eyepiece focal length, then play with the FoV until what the calculator shows you matches what you saw) To calculate it then: cloudynights.com/topic/544944-how-do-you-measure-afov/… $\endgroup$
    – Aaron F
    Jan 20, 2022 at 17:36


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