function pop_up_center(url, name, w, h) {
  l = (screen.availWidth-10 - w) / 2;
  t = (screen.availHeight-20 - h) / 2;

  features = "width="+w+",height="+h+",left="+l+",top="+t;
  features += ",screenX="+l+",screenY="+t;
  features += ",scrollbars=0,resizable=0,location=0,directories=0";
  features += ",menubar=0,toolbar=0,status=0";

  window.open(url, name, features);
}