var name = "";
    var fenster = null;
	var breite = 0;
	var hoehe = 0;

	function auf(datei,breite,hoehe)     {
    var xpos = (screen.width-breite)/2;
	var ypos = (screen.height-hoehe)/2;
    
    fenster=window.open( datei,'Impressum','locationbar=no,menubar=no,toolbar=no,status=no,scrollbars=yes,resizable=yes,width='+breite+',height='+hoehe+',screenY='+ypos+',screenX='+xpos+',top='+ypos+',left='+xpos);
    }
	
	function zu() {
        if(fenster == null)  {  
           return ;
        }
        if(fenster.closed == true) {  
           return (fenster = null);
        }
        else	{ 
          fenster.close();
          fenster=null;
        }
    }