function open_centred_window(page, window_width, window_height) {
   var window_left_offset = (screen.availWidth-window_width)/2
   var window_top_offset = (screen.availHeight-window_height)/2
   var window_configuration="height=" + window_height + ",width=" + window_width + ",left=" + window_left_offset + ",top=" + window_top_offset + ",toolbar=no,menubar=no,resizable=no,scrollbars=no,status=no"
   //var window_configuration="height=525 width= 420, left=190, top=0"
   window.open(page,"",window_configuration)
}