... The initial descriptions of the FITS format provided a simplified method for describing the physical coordinate values of the image pixels, but deliberately did not specify any of the detailed conventions required to convey the complexities of actual image projections. ... They were CRPIX n for the reference pixel location on pixel axis n , CRVAL n for the coordinate value at that pixel, CDELT n for the increment at that pixel in the coordinate value, and CTYPE n for the type of coordinate. ...