code: def sky_to_offset(skydir, lon, lat, coordsys='CEL', projection='AIT'):
      
  
      w = create_wcs(skydir, coordsys, projection)
      skycrd = np.vstack((lon, lat)).T
  
      if len(skycrd) == 0:
          return skycrd
  
      return w.wcs_world2pix(skycrd, 0)
docs: Convert sky coordinates to a projected offset.  This function
      is the inverse of offset_to_sky.