var popup = false; 

function openpopup(url,w,h) {

x = screen.width/2-w/2;
y = screen.height/2-h/2;

	if ( popup ) popup.close();
    popup=window.open(url,'ny','toolbar=no,copyhistory=no,location=no,' + 'directories=no,status=no,menubar=no,scrollbars=no,' +'resizable=no,width=' + w + ',height=' + h + ',top=' + y +',left=' + x);
};