13mm Nagler T6 Question

I just bought a 13mm Nagler Type 6 and tried it out the first time last night on my MN56 5" F6 Mak-Newt. I noticed right away observing Jupiter at twilight that the field started to get darker about 95% out to the edge. This was very noticeable, looking like a dark rim around the edge. I attributed this to vignetting, so I tried it on my XT-10, and it was not as bad but still there. I don't see this on my 7mm t6, or other eyepieces. Has anyone noticed this? Is it normal, or is my eyepiece defective?

Thanks!
Alan

Alan,
I agree with Clyde, no issues like that in my TV101 and SM11 f/5.4, in fact it' one of my favorite eyepieces.
Buck

Prompted by your note I popped some of the Nagler Type 6
in my 6" F6 Mak-Newt. Indeed I see exactly the same thing.
Both the 11mm and 13mm show this effect, but not the
9mm or 7mm. If you look at the bottoms of the eyepieces,
you will see that the 11 and 13 are a different optical
design, with the first surface being convex. I think
it is probably some fine detail of the F-ratio and the
trajectory of light through the eyepiece -- either one
element is not quite large enough to accomodate all trajectories,
or there is some element with a steep
curve that starts to total-internal reflect the rays
right at the edge. I would not worry about it.

Thanks everyone. I called Tele Vue today and talked with a tech person. He said it's due to the scope and asked if I could see the darkening just looking through the eyepiece with no scope, and replied I could not. The effect is much less apparant on my XT-10, but still! I never thought vignetting would be a problem at 13mm. He also assured me that each eyepiece is completely tested, and if he thought something was wrong, he would have had me send it in for inspection. I will have to compare it with another one. This is a great eyepiece otherwise.

Thanks again
Alan