"gregjohn" <pte### [at] yahoocom> wrote:
> 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?

You'll find that the ring splits into two arcs for a real source, yes, because
the lensed source will have a finite size. But if you take any single point on
the source, you'll find that there are no more than two points that correspond
to it on the lensed image - except for the source point that's directly behind
the super-dense ball (you understood correctly), which maps to a complete ring
in the lensed image due to the symmetry.

You can model the lens effect with an appropriately shaped glass lens. The
bottom of a wineglass can be quite close to the required shape, you need
something roughly conical (or more steeply sloped if possible). Or you could
use POV, just create a conical lens with its apex facing towards you and move a
bright sphere behind it. Annoyingly none of the interactive simulations (java
and such) I used to be aware of seem to be around any more.

Tom

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