Center of the image plane is the mirror axis. Can be input or output. The focal plane Y pixel coordinate. See the CXC Coordinates papers for definitions. Gives position on the tangent plane to the mirror optical axis.
Pixel size is fixed in angular size.
Center of the image plane is the mirror axis. Can be input or output. The sky plane X pixel coordinate. See the CXC Coordinates papers for definitions. Gives position on the tangent plane to the nominal celestial pointing direction.