function popup(url,breite,hoehe) {
            nleft = (screen.width-breite)/2;
            ntop  = (screen.height-hoehe)/2;
            var PopUp = window.open(url,"popup","height=" + hoehe
                                              +",width=" + breite
                                              +",top="+ ntop
                                              +",left="+ nleft
                                              +",resizeable=no,"
                                              +",location=no,"
                                              +",menubar=no,"
                                              +",status=no,"
                                              +",scrollbars=yes,"
                                              +",toolbar=no");
            PopUp.focus();
    }