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Dave,
Unfortunately specifying an F5 scope is only specifying 1/2 the equation because we also need to get a handle on the scopes focal length. ie The same eyepiece that is suitable for planetary observation in Andrew Murrell's 20"/F5 is not suitable for planetary observation in a 6"/F5.
However, assuming an F5 scope with a focal length of about 1250mm (10"/F5), my 2 ideal eyepieces would be a 7mm Pentax XW and a 5mm Pentax XW. These perform exceptionally well at F5, have a wide and flat FOV for use in an undriven scope and are very sharp with high light transmission and have comfortable eye-relief of 20mm.
The Zeiss orthoscopics are the best planetary eyepieces I have used in terms of light transmission, contrast and sharpness but they are limited by the focal lengths they come in and their short eye-relief at short focal lengths. However, in a scope of about 1200mm focal length, the 6mm zeiss ortho is as good as it gets.
CS-John B
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