= Tangential critical curve of an axisymmetric lens
{title2=$\lambda_t=1-\bar\kappa=0$}
For circular reduced deflection $\alpha(r)=r\bar\kappa(r)$, a displacement perpendicular to the radial direction is multiplied by $1-\bar\kappa$. Vanishing of this <eigenvalue> gives a tangential <gravitational-lensing critical curve>. Its source position is $\beta=r-\alpha(r)=0$, so its caustic is the origin in the circular model. Near it, images are stretched tangentially into arcs or an <Einstein ring>.
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