= Cluster gravitational-lensing mass estimate
{title2=$\kappa=\Sigma/\Sigma_{\rm crit}$}
<Gravitational lensing> constrains a <galaxy cluster>'s projected mass independently of its light output or dynamical equilibrium. The <lensing convergence> is projected <surface mass density> divided by $\Sigma_{\rm crit}=c^2D_s/(4\pi GD_dD_{ds})$, using the appropriate <angular diameter distances>. <Strong gravitational lensing> constrains the central mass through multiple-image geometry, and <weak gravitational lensing> constrains larger-scale projected mass through coherent image distortions. Converting these observations into a three-dimensional mass needs geometrical modelling and the relevant lensing degeneracies.
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