function showhilfe(thema) {
  var dummy  = "";
  var width  = 480;
  var height = 560;
  var top    = (screen.availHeight - height)/2;
  var left   = (screen.availWidth  - width )/2;
  dummy = window.open('./help/'+thema,'showhilfe','scrollbars=yes,toolbar=no,location=no,directories=no,status=no,menubar=no,resizable=yes,copyhistory=no,width=' + width + ',height=' + height + ',left=' + left + ',top=' + top);
}