Most EP manufacturers list focal length and apparent field. You can
calculate the field stop (or at least a close approximation) by: field stop
in mm = (EP focal length) X (AFOV in degrees) / 57.3.
This is >38mm only for very low power wide field (ie, widest TFOV) 2" EPs.
-John


>
>I'm not Mike, but here's my take on the Eyeopener:
>
>It won't make any difference with your CCD camera.
>
>With regard to eyepieces, the limiting factor for light throughput is not
>necessarily the scope's standard visual back diameter (1.5" or 38.1-mm) or
>the
>2" diagonal's diameter (~48-mm). It is the diameter of the field stop in
>the
>eyepiece. The diameter of the optical path only needs to be equal to or
>larger
>than that to get all of the available light to your eye. Currently, out of
>Tele
>Vue's entire eyepiece line-up, only four have field stop diameters larger
>than
>38.1-mm. If you don't use one of those four, the Eyeopener will only
>function to
>lighten your wallet. See the Tele Vue eyepiece chart here:
>
>http://www.televue.com/engine/page.asp?ID=144
>
>Other manufacturers may also have eyepieces with field stops wider than
>38.1-mm,
>but I can't find specification charts for them.
>
>Regards,
>
>Don
>
>-----Original Message-----
>
>Hey Mike,
>
>How's the EyeOpener? Do you really notice a difference? I suppose it only
>helps with 2" EPs, so it wouldn't make any difference with my 1.25" CCD
>camera.
>Correct?
>
>Christian.
>

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