function Fenster(sitename,myWidth,myHeight) 
 { var isCenter = 'true';
  if(window.screen)if(isCenter)if(isCenter=="true"){
    var myLeft = (screen.width-myWidth)/2;
    var myTop = (screen.height-myHeight)/2;
    var features =',left='+myLeft+',top='+myTop;
  }
  Popup=
      open(sitename,"fenster",features+((features!='')?',':'')+'width='+myWidth+',height='+myHeight);
  Popup.focus();  
}