code: def zoomset_cb(self, setting, value, channel):
          
          if not self.gui_up:
              return
          info = channel.extdata._info_info
          if info is None:
              return
          #scale_x, scale_y = fitsimage.get_scale_xy()
          scale_x, scale_y = value
  
          # Set text showing zoom factor (1X, 2X, etc.)
          if scale_x == scale_y:
              text = self.fv.scale2text(scale_x)
          else:
              textx = self.fv.scale2text(scale_x)
              texty = self.fv.scale2text(scale_y)
              text = "X: %s  Y: %s" % (textx, texty)
          info.winfo.zoom.set_text(text)
docs: This callback is called when the main window is zoomed.