Weak Lensing Observables
In weak gravitational lensing, the Jacobian is mapped out by observing the effect of the shear on the ellipticities of background galaxies. This effect is purely statistical; the shape of any galaxy will be dominated by its random, unlensed shape, but lensing will produce a spatially coherent distortion of these shapes.
Read more about this topic: Gravitational Lensing Formalism
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—William Butler Yeats (1865–1939)