How far above the horizon the parent sits by default, radians.
Three degrees. Apollo 8's frame has Earth a little under two disks above the limb, and Earth is 1.9° across from Luna — so a few degrees is the picture. Lower and the parent is cut by the terrain the phase-2 geology will put there; higher and the horizon leaves the frame, at which point it is a photograph of a planet with nothing to say where it was taken from.
rise.ts:50