javascript让弹出的窗口永远居中

function ShowDialog(url)
{
  var  iWidth=600; //模态窗口宽度
  var  iHeight=500;//模态窗口高度
  var  iTop=(window.screen.height-iHeight)/2;
  var  iLeft=(window.screen.width-iWidth)/2;
  window.open(url,"Detail",
 "Scrollbars=yes,Toolbar=no,Location=no,Direction=no,Resizeable=no,   Width="+iWidth+" ,Height="+iHeight+",top="+iTop+",left="+iLeft)
}