The last of the Frontier Fields - Abell 370

With the final observation of the distant galaxy cluster Abell 370 – some five billion light-years away – the Frontier Fields program came to an end.
 
Abell 370 is one of the very first galaxy clusters in which astronomers observed the phenomenon of gravitational lensing, the warping of spacetime by the cluster's gravitational field that distorts the light from galaxies lying far behind it. This manifests as arcs and streaks in the picture, which are the stretched images of background galaxies.
 
Read more: The final frontier of the Frontier Fields [heic1711]