A Chang–Refsdal lens is a point-mass gravitational lens (e.g. black hole) perturbed by constant external shear.
[1] The name derives from Kyongae Chang and Sjur Refsdal who in 1979 published a paper in NATURE 282, 561.
"Flux Variations of QSO Q0957+561 A,B and image splitting by stars Near the Light Path."
The paper illustrated that stars could affect quasar image brightness.
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