function popup(url) {
    var width = 820;
    var height = 450;
    var popupLeft = (screen.width  - width)/2;
    var popupTop = (screen.height - height)/2;
    var params = 'toolbar=no, location=no, status=no';
    params += ', height=' + height;
    params += ', width=' + width;
    params += ', top=' + popupTop;
    params += ', left=' + popupLeft;
    params += ', resizable=0, menubar=0, scrollbars=no';
    var newwindow = window.open(url, 'tara', params);
}