I recently got the televue eyeguard extender for a Vixen 42 mm LWR eyepiece ... it fits and seems to work well, a much better eye position.
However I noticed that with the increased distance, looking into the eyepiece I no longer see the sharp edge which I assume corresponds to the field stop. This was an eyepiece-only test against a bright background.
I performed the same experiment with my other eyepieces, which include a televue pan 24, TV radian 14 mm, and 11 mm Plössl, also Meade 55 Plössl and the Meade 40 superwide that the Vixen is replacing. In nearly every case I can only see the sharp outline of the field stop when I press my eye fairly deep into the eyecups, deeper than I use observing. Or only by moving the eye to one side can I glimpse the field stop.
Never noticed this in the dark observing before, but the effect might explain why the actual FOVs that people report may be a bit narrower than the specification for that EP.
Comments, anyone ... has anyone noticed a similar effect?
When you focus in a telescope, you are focusing at a distance, and since the field stop is at or near the focal plane, you see it in focus.
When you look through an eyepiece at the sky, there is no way to know what focal point your eye takes, but if you look directly at the field stop, it is likely to be seen with a close-up focus.
If you are young, it may still be in focus. If you're older, it likely won't be.
That does not mean you won't see it in focus in the scope.
Also, your daylight pupil will be quite small. To field the image from the eyepiece, you will have to avert your gaze to see the edge of the field.
At night, with a larger pupil, the field stop will be more inside your field of vision because the entire exit pupil will be.
To check for the field stop focus, always use it in a telescope at night.
A couple things you can check for looking through an eyepiece at the sky:
--lateral field vignetting (the edge is darker than the center) Note: most long focal length eyepieces will have some.
--lateral field color/tinting, like the "ring of fire" in the outer 10% of the field in the 31 Nagler and 30mm ES.
--dirt particles at/near the focal plane
--reflective surfaces inside the eyepiece (spacers, lens edges, etc) that could be a problem at night. If you see the threads at the bottom of the eyepiece, blacken them with flat black paint.
Otherwise, bright stars just outside the field at night may cause a glare artifact inside the FOV.