function open_centered(file, width, height) {
    var x, y;
    x = screen.availWidth / 2 - width / 2;
    y = screen.availHeight / 2 - height / 2;
    var win = window.open(file, 'centered', 'width=' + width + ',height=' + 
    height + ',screenX=' + x + ',screenY=' + y + 
    ',location=no,,menubar=no,resizable=no,scrollbars=no,status=no,' + 
    'toolbar=no');
    win.focus();
    return win;
}