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