Multiple images in gravitational lensing

[Angelos K]

Dear all,

so far I have only studied weak lensing. Therefore I understand little about multiple images.The light paths, by Fermat's principle, have to correspond to stationary points of the (proper) time delay. Minima, maxima saddle points. Basics are nicely reviewed here [p.14]:
http://www.ita.uni-heidelberg.de/research/bartelmann/Lectures/EinfSeminar/JeruLect.pdf

My question is why saddle points and maxima correspond to multiple images as for example claimed here [p.11]:
Takahashi et al. 2011
Am I right to assume that each maximum or saddle point creates two images?

All the best,
Angelos

 

[Adrmay]

Yes, you are right to assume that each maximum or saddle point creates two images. This is because the light paths of multiple images must converge at the critical points (maxima and saddle points). The saddle points correspond to two distinct images that have different brightness and orientation, whereas maxima correspond to two identical images that have the same brightness and orientation. Therefore, multiple images can be formed when light passes through a critical point such as a maximum or saddle point.