如何使弹出窗口居中显示

function add_qc_object(){
var height = 250;
var width = 800;
var top = (screen.availHeight - height)/2;
var left = (screen.availWidth - width)/2;
var param = 'toolbar=no,location=no,directories=no,status=yes,menubar=no,scrollbars=yes,resizable=yes,' +
'width=' + width + ',height=' + height + ',left= ' + left + ',top=' + top;
window.open('K230_add_qc_object.jsp', 'win_name', param);
}