function showcert() {
  var loc = 'cert.php';
  var ah = screen.availHeight;
  var aw = screen.availWidth;
  var wh = 600;
  var ww = 460;
  var ll = (ah - wh) / 2;
  var rr = (aw - ww) / 2;
  window.open(loc,'CERT','scrollbars=yes,toolbars=0,width='+ww+',height='+wh+',left='+rr+',top='+ll+'screenX='+rr+',screenY='+ll);
}