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4 Coordinate Systems

Source sky positions are specified in cylindrical coordinates, with the Z axis along the optical axis. The angular distance from the optical axis is described by theta, the azimuth about the axis by phi, where phi = 0 is along the +X axis. The distance to the source is actually specified by its Z coordinate, which is generally zero at the telescope’s node, except in the case of the AXAF HRMA, where it is zero at CAP datum A. (The ray generator actually doesn’t set the zero point; it just cares about where the node is.)