Standard Co-Ordinates in Celestial Photography

[Taking Prof. Turner's definition of standard co-ordinates or Co-ordinates on a standard or ideal photographic plate, let C be the projection on the sky of the centre of the photographic plate, S a star, P the pole, and p the position angle of C S. Let O be the second principal point of the objective (the centre of projection), c the centre of the plate, s the image of the star on the plate having standard co-ordinates x, y].