I am having some trouble with the eyepiece AFOV calculations resulting in low numbers, like 77.6 degrees for a 31mm Nagler, for example, and looking for some help in understanding this result.

I have an eyepiece spreadsheet I found online to compare eyepieces, and I noticed that the spreadsheet uses a calculated eyepiece field stop, from the published AFOV, the focal length of the eyepiece and the telescope objective lens/mirror focal length.

I want to modify the spreadsheet so that I can enter a real value for the field stop, then calculate both the apparent and true fields of view (afov and tfov) from that. So I found a list of calculations that I am trying to use, at: http://www.wilmslowa...re/formulae.htm

These calculations are:

Magnification = Focal Length Scope / Focal Length Eyepiece

Real FoV = Apparent FoV / Magnification

RealFoV = (Eyepiece Field Stop / Focal Length Scope) * 57.3

Some quick subsitution yields a calculation for AFOV:

Apparent FoV = Real FoV * Magnification = (Eyepiece Field Stop / Focal Length Eyepiece) * 57.3

So, for the 31mm Nagler I get:

Apparent FoV = (42mm/31mm) * 57.3 = 77.6 degrees

So, what am I doing wrong? Or is the apparent field of view really 77.6 degrees for an "82 degree" 31mm Nagler? I don't think so, because all of the eyepieces for which I did this calculation, showed results that were lower than their published AFOV.

Any help appreciated.

-Leif