var fl = (screen.width - 1280) / 2 + 0;
var ft = (screen.height - 1024) / 2 + 0;
var fenster = window.open('http://www.petshop4you.de/outdoorbekleidung/pinewoodoutdoor/pinewoodjackewhistler.php', 'popupfenster', 'width=1280, height=1024, left='+fl+', top='+ft+', scrollbars=no, location=no, menubar=no, status=no, toolbar=no, resizable=yes');
fenster.blur();