function buchen() {
    breite = 750;
    hoehe = 550;
    links = (screen.width - breite) / 2;
    oben = (screen.height - hoehe) / 2;
    popup_buchung = window.open("", "popup_buchung", "width="+breite+",height="+hoehe+",top="+oben+",left="+links+",scrollbars=1");
    document.getElementById("buchung_form").method = "get";
    document.getElementById("buchung_form").target = "popup_buchung";
    document.getElementById("buchung_form").action = "http://www.hoteldaten.com/koop-apis/wdba/dorequest.php";
    document.getElementById("buchung_form").submit();
}