function popup(url, width, height) {
        var windowX = Math.ceil( (window.screen.width-width) / 2 );
        var windowY = Math.ceil( (window.screen.height-height) / 2 );
        window.open(url,'popup','width=' + width + ',height=' + height +',resizable=0,scrollbars=no,menubar=no,left='+windowX+',top='+windowY);
}