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David Knisely
Postmaster
Reged: 04/19/04
Posts: 8266
Loc: Beatrice, Nebraska
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Re: Strange Results in Eyepiece FOV Comparisons
05/17/08 05:46 AM
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I hate to say it again, but it bears repeating: The old TRUE FIELD = APPARENT FIELD divided by the MAGNIFICATION is *only* an approximation (10 percent accuracy usually). There are numerous instances where it just plain falls apart. You *cannot* use the formula backwards to get a very accurate apparent field of view figure! The only way to get accurate figures for apparent fields of view is to actually measure them on the optical bench (there are a few threads on Cloudynights which outline the details, and it isn't terribly hard to do). The apparent field of view is the angle which your *eye* sees when looking in the eyepiece (whether it is in a telescope or not). It is what it is regardless of what any formula or calculation says it is. For true field figures, the drift method is the way to go for certain. For a good formula for true field of view, the field stop formula can give fairly decent results (within a percent or two), but again, you just can't work backwards to get an apparent field of view figure that means very much. You have measured the true field of view that your eyepieces produce in your scope, and that is really all that matters. The apparent field is just the "size of the window" you are looking through, so it often isn't nearly as important as some people make it out to be. Clear skies to you.
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