Oblique viewing phenomenon?

Hi, all,
I have been experiencing a strange eyepiece phenomenon, in a variety of eyepieces, in a variety of Dobsonian reflectors, and this has been seen by two other experienced observers in their own telescopes with their own eyepieces.
I think this rules out factors related to a specific observer, or a specific eyepiece, or a specific telescope, or collimation.
What I see when I look at Jupiter in the center of the field of view of my eyepiece (I use mostly Ethoi or Naglers) and my eye centered over the center of the eyelens of the eyepiece, is a circular glow surrounding the planet, but when I position my eye so that I am looking at Jupiter through one side of t he eyepiece at an oblique angle, Jupiter is surrounded by black sky.
Any suggestions as to causation?
Thanks,
Bill

It could be also influenced by the shift of the image formed on the retina, which inevitably follows tilting the eye w/respect to the eyepiece axis. Looking at the distribution of retinal photoreceptors

http://www.telescope-optics.net/eye_spectral_response.htm

(Fig. 155), two different scenarios seem possible. If we intuitively tilt the eye so that the image formed by eye lens falls onto the center of vision, the halo might be showing due to high cone density and the ensuing maximum level of (bright) light absorption. Tilting the eye shifts the image onto retinal area of significantly lower cone density, while the rods are still too few and/or sufficiently deactivated by bright object to produce appreciable visual effect.

Alternately, if we don't intuitively bring the image to the center of vision, we might be seeing due to the presence of low-ligh-sensitive rods away from it (they are certainly active to some extent), and lose it by bringing the image near the center of vision. Obviously, in this scenario the halo would disappear only when tilting eye in one particular direction.

Vla

Bill,

I have also seen this several times with my Ethos.. but in the case with me it was moisture build-up on the eyelens itself, I simply back off and fan the eyepiece a few seconds with my hand and it dissipates. Heat from my eye, or a combination of heat from the eye and cupping around the eyepiece itself to sheild off stray light may explain the moisture build-up (especially during observing when the moon is so bright)

Mark