def proj_plane_pixel_scales(wcs):
 return np.sqrt((wcs.pixel_scale_matrix ** 2).sum(axis=0, dtype=np.float))

def proj_plane_pixel_scales ( wcs ) : return np . sqrt ( ( wcs . pixel_scale_matrix ** 2 ) . sum ( axis = 0 , dtype = np . float ) )
for a wcs returns pixel scales along each axis of the image pixel at the crpix location once it is projected onto the "plane of intermediate world coordinates" as defined in greisen & calabretta 2002 .