Warp <war### [at] tagpovrayorg> wrote:
> gregjohn <pte### [at] yahoocom> wrote:
> > I think that gravitational lensing makes more intuitive sense as a ring
>
>   I wonder if the shape of the lensing object makes a difference. After all,
> galaxies are seldom spherical.
>
> --
>                                                           - Warp

Yes, the shape of the projected mass distribution of the galaxy alters the
number, location and distortion (both angular size/magnification, and shape) of
the images.

A galaxy whose projected mass distribution has pure circular symmetry can
produce three distinct images (per source) when the source is away from the
centre of mass (but is still within a critical distance of the centre), and
will produce an Einstein ring when some part of the source falls directly
behind it.

A projected mass distribution that has a degree of ellipticity can produce five
distinct images (per source) when the source is close enough to the centre of
mass, or three images as the source is moved far enough away from the centre
(but is still kept within a critical distance).

The third and fifth images form close to the centre of mass (and light), and
unlike the other images are strongly demagnified instead of boosted by the lens
effect. This means that they are usually not observable, and it's common to talk
of two- or four- image systems as a result.

Tom

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