gregjohn nous apporta ses lumieres en ce 2008/01/16 07:10:
> Okay, I'm back to square one.  I'm bothered by getting any points at all. I want
> rings.
> 
> Say your interposed object is a super-dense ball (in case I don't understand
> "circular symmetry")  in front of a star, then you would get a ring.  If it's
> off to the side, don't you get a distorted ring instead of points?
> 
> Unless the magnetism of the interposed ball has an effect on the light, or the
> polarized photons from the star are affected by gravity in some way, you should
> get a band of light and not discrete points.  Or maybe magnetism of the
> intervening ball affects how its material is "available" for lensing, making an
> assymetrical lens.  There's gotta be one more phenomenon at work besides
> gravity.
> 
>  
If the lencing object is precisely on the line from the observer and the lensed 
object, you get a ring.
If the lencing object is slightly shifted, you get one or more cressent(s) and 
possibly a specular dot.
If the lecing object is even more shifted sideway, you see a slightly shifted 
primary image and a lenced punctual or arced image.

If the lencing object is homogeneous, the effect is regular.
If it is diffuse and somewhat irregular, like a galaxy, the effect become 
distorded. The more irregular, the more distortion you get.

-- 
Alain
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Never delay the ending of a meeting or the beginning of a cocktail hour.

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