function centered_popup(URL, wwidth, wheight)
   {
	  var wleft = (screen.width-wwidth)/2;
	  var wtop = (screen.height-wheight)/2;
	  window.open(URL, '_blank',
		'toolbar=no,location=no,titlebar=yes,status=yes,menubar=no,'+
		'resizable=no,copyhistory=no,scrollbars=yes,'+
		'width='+wwidth+',height='+wheight+',top='+wtop+',left='+wleft);
   }