var windowW=500 // wide
var windowH=500 // high
var windowX = (screen.width/2)-(windowW/2);
var windowY = (screen.height/2)-(windowH/2);

function popup (url) {
 var fenster=window.open(url, "fenster1", "width="+windowW+",height="+windowH, "status=no,scrollbars=no,resizable=no");
 fenster.moveTo(windowX,windowY);
 fenster.focus();
}