A Chang-Refsdal lens is a point-mass gravitational lens

Gravitational lens

A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...

 (e.g. Black hole

Black hole

A black hole is a region of spacetime from which nothing, not even light, can escape. The theory of general relativity predicts that a sufficiently compact mass will deform spacetime to form a black hole. Around a black hole there is a mathematically defined surface called an event horizon that...

) perturbed by constant external shear.

The name derives from Kyongae Chang

Kyongae Chang

Kyongae Chang is a South Korean astrophysicist. She is best known for her work on gravitational lensing, including the Chang-Refsdal lens.Chang was born in Seoul. She worked as a research associate on astrometric binaries with Professors van de Kamp and Heintz at Sproul Observatory from 1969 till...

 and Sjur Refsdal

Sjur Refsdal

Prof Sjur Refsdal was a Norwegian astrophysicist, born in Oslo. He is best known for his pioneer work on gravitational lensing, including the Chang-Refsdal lens....

 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

Quasar

A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...

image brightness.