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Lawrence Sayre
Abbe Normal
Reged: 10/16/04
Posts: 4731
Loc: N.E. Ohio
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Re: Strange Results in Eyepiece FOV Comparisons
05/17/08 06:38 AM
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Your results seem to strongly vindicate the concept of the RFOV ("Real Field Of View", formerly known as eAFOV, which stood for "effective Apparent Field Of View"). They also verify that unlike virtually all other eyepieces which have AFOV enhancing and thereby TFOV robbing pincushion distortion, the Hyperions (some at least, and perhaps all of them) have AFOV robbing and TFOV enhancing barrel distortion.
Also, the field stops for the TeleVue eyepieces would indicate exactly the TFOV's that you have measured. A powerful vindication of the field stop method of measuring TFOV, and (as David has indicated already) strong proof that the TFOV = AFOV/Mag method is not very precise (as opposed to TFOV = RFOV/Mag being right on the money,and TFOV = FS/Telescope_FL * 180/Pi also being right on the money).
I'm the worlds first (and perhaps only) advocate of urging eyepiece manufacturers, distributors, and retailers to replace their reporting of the very misleading and generally impractical AFOV with the quite useful and practical RFOV.
Provided that the focal length of an eyepiece is exactly as indicated, the two following formulas are true, and they are also equivalents:
RFOV = FS/Eyepiece_FL * 180/Pi
RFOV = AFOV * (1 + FD)
[where FD = the fractional representation of the aggregate percentage of pincushion and/or barrel type field distortions present within the eyepiece, with pincushion being negative in value, and barrel being positive in value. For example, 4% pincushion distortion would mean that FD = -0.04]
Due to inherent field distortion, this statement is false:
TFOV = AFOV/mag
Whereas this statement is true:
TFOV = RFOV/Mag
So therefore this statement (a third definition of RFOV) is also true:
RFOV = TFOV * Mag
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My philosophy, in essence, is the concept of man as a moral being, with his own happiness as the moral purpose of his life, with productive achievement as his noblest activity, and reason as his only absolute.
Ayn Rand (in the appendix to 'Atlas Shrugged')
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