function popup(url, width, height, centered) {
    width = (width) ? width : 800;
    height = (height) ? height : 600;
    var left = 10 ;
    var top = 10 ;
    if (centered) {
	  left = (screen.width-width)/2;
	  top = (screen.height-height)/2;
	  if (left < 0) left = 10;
	  if (top < 0) top = 10;
    }
    var eigenschaften = "scrollbars=yes, left="+left+", top="+top+", width="+width+", height="+height+", resizable=yes";
    popupWin = window.open(url, '', eigenschaften);
    if (popupWin) { if (popupWin.focus) { popupWin.focus(); } }
    return false;
}//end function