Below this dialog there are 3 sets of eyepiece calculations for "low", "mid" and "hi" powers. Each section provides the option to select a commercial brand and model of eyepiece as well as a Barlow power to be used with them.
Initially, an exit pupil size of 6mm (default) determines the theoretical "low" power eyepiece mm focal length recommended for the specified primary objective. The "mid & hi" power EP values are then determined based on even multiples of the "Barlow" power(s) selected. If only one Barlow is to be used with all 3 categories, enter the x value in the low power Barlow field (defaults to 2x).
Keep in mind that an eyepiece selection resulting in an exit pupil size greater than 7mm (considered the typical maximum dilated human iris) may cause some edge vignetting of the available true field provided by the eyepiece.
When the "calculate" button is clicked BEFORE selecting and "using" an eyepiece model, the model having the nearest mm size for the selected eyepiece brand in each category will be displayed in the "EP CHOICE" field. If a specific eyepiece is then selected in one or more categories (low, mid & high power) and activated with the associated "USE" checkbox, you must recalculate to obtain the correct associated values for the selected eyepiece(s). Also, if you change eyepiece brands, you must recalculate to display the nearest size eyepiece for the new brand.
The eyepiece field stop diameter (FSD), true field of view (TFOV), effective field of view (EFOV), apparent field of view (AFOV), telescope focal length (TFL) and eyepiece focal length (EPFL) are related by the following equations: TFOV=FSD*57.3/TFL; EFOV=FSD*57.3/EPFL; EFOV=TFOV*(TFL/EPFL). When the FSD is published (i.e. TeleVue), an EFOV & TFOV can be accurately calculated. When the FSD is unknown (as is the usual case), the published AFOV (which is generally inflated over the EFOV) is substituted for the EFOV resulting in a corresponding "inflated" calculated FSD.
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LOW POWER EYEPIECE DATA
LOW EP FL (mm):
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(MAX) EXIT PUPIL (mm):
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ScopeX:
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SELECT A BRAND:
TeleVue
Takahashi
Pentax
Meade
EP CHOICE:
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USE
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APP. FOV (deg):
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E.R.(mm):
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FIELD DIA (mm):
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TRUE FOV (deg):
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(arc min):
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WITH A
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X BARLOW:
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EXIT PUPIL (mm):
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ScopeX:
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APP. FOV (deg):
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TRUE FOV (deg):
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(arc min):
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