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The Image Display, or North/South (NS), system is a cartesian frame ( ) optimised for use by image display S/W. By convention, astronomical images are usually displayed in the same sense as one would see from the Earth (i.e., North to the top, East to the left). However, image display devices usually display images from top-to-bottom and from left-to-right. Thus the transformation from the relative coordinate system ( ; Sect. 2.5.2) to the NS system is (Fig. 2.7):
(the -axis is aligned with the nominal pointing direction of the S/C, as is the case for the -axis).