Figure 7: Thin bundle around a ray in a spherically symmetric static spacetime. The picture is purely spatial, i.e., the time coordinate is not shown. The ray is contained in a plane, so there are two distinguished spatial directions orthogonal to the ray: the “radial” direction (in the plane) and the “tangential” direction (orthogonal to the plane). For a bundle with vertex at the observer, the radial diameter of the cross-section gives the radial angular diameter distance , and the tangential diameter of the cross-section gives the tangential angular diameter distance . In contrast to the general situation of Figure 3, here the angle is zero (if the Sachs basis is chosen appropriately). Recall that and are positive up to the first caustic point.