function popup(url,fenster,width,height,center){
	x=screen.availWidth/2-width/2;
	y=screen.availHeight/2-height/2;
	args='width='+width+',height='+height+',screenX='+x+',screenY='+y+',scrollbars=no';
	fenster = window.open(url,fenster,args);
	if(center) fenster.moveTo(screen.availWidth/2-width/2,screen.availHeight/2-height/2);	
}