code: def zoomset_cb(self, setting, zoomlevel, fitsimage):
          
          if not self.gui_up:
              return
          fac_x, fac_y = fitsimage.get_scale_base_xy()
          fac_x_me, fac_y_me = self.zoomimage.get_scale_base_xy()
          if (fac_x != fac_x_me) or (fac_y != fac_y_me):
              alg = fitsimage.get_zoom_algorithm()
              self.zoomimage.set_zoom_algorithm(alg)
              self.zoomimage.set_scale_base_xy(fac_x, fac_y)
          return self._zoomset(self.fitsimage_focus, zoomlevel)
docs: This method is called when a main FITS widget changes zoom level.