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No one doubts that the AFOV's are as reported. They simply don't matter. All that matters is the RFOV and/or the field stop diameter. The AFOV will not tell you how much sky you will see. The RFOV and the field stop will tell you how much sky you will see. And the RFOV (or the field stop) will tell you without measurements of any nature, provided that the manufacturers get on the stick and accurately report these values.

I would most certainly like to know more about your method though!!!

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The Apparent Fields *do* matter, especially to those who care how much they see. Indeed, with a few of my eyepieces, the apparent field used in the old AFOV/Mag calculation gave results that were *very* close to the true fields I actually measured. The problem is that the formula just isn't consistently accurate, which is why the field stop formula tends to be a bit better. The apparent field is real, as it can be seen and measured. The Field stop is real and can be measured (usually). The true field of view of a telescope/eyepiece combination can be measured. The RFOV is not a real characteristic of the eyepiece. It is an artificial construct to get an old approximate equation for true field to seem to work better. It already involves the field stop in its calculation, which is redundant. All that really matters is that if someone wants to know *exactly* how much true field an eyepiece will give in a telescope, it needs to be *measured* (star drift method, or an accurately-known field for comparison).

If the amateur wants to know roughly how much true field one can get with a given eyepiece with known physical parameters, there are *two* formulae which work. The old formula TFOV = AFOV/Magnification will give at least an approximate result (within 10 %) and the field stop formula will give somewhat closer results (within 2%). Introducing another concept that has less of a clear connection to reality might be confusing and is unnecessary, considering we already have two workable formulae for true field of view. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info
Prairie Astronomy Club
http://www.prairieastronomyclub.org

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