Bipolar Coordinates
A given blue circle and a given red circle intersect in two points. In order to obtain bipolar coordinates, a method is required to specify which point is the right one. An isoptic arc is the locus of points X that sees points C and D under a given oriented angle of vectors i.e.
Such an arc is contained into a red circle and is bounded by points C and D. The remaining part of the corresponding red circle is . When we really want the whole red circle, a description using oriented angles of straight lines has to be used
Read more about this topic: Apollonian Circles