Radial critical curve of an axisymmetric lens
= Radial critical curve of an axisymmetric lens
{title2=$\lambda_r=1-2\kappa+\bar\kappa=0$}
The local and mean <lensing convergence> obey $2\kappa=2\bar\kappa+r\bar\kappa'$. Hence the radial <derivative> of the lens mapping is $1-\alpha'(r)=1-2\kappa+\bar\kappa$. Its zeros are radial <gravitational-lensing critical curves>, where radial stretching diverges and images merge on crossing the mapped <gravitational-lensing caustic>.