Coordinate system used in calculating the acceleration field of clusters in the plane of the sky. The blue plane shows the plane of the three clusters that define the coordinate system. The x₂ coordinate direction connects A2089 and A2065, the y₂ coordinate is perpendicular to x₂ and is in the same plane as determined by the clusters. The z₂ coordinate is perpendicular to both x₂ and y₂.