function open_pop_up(url, width, height) {
    left_position = (screen.width) ? (screen.width - width)/2 : 100;
    top_position = (screen.height) ? (screen.height - height)/2 : 100;
    window.open(url, "docPopup", "toolbar=no, location=no, directories=no, status=no, resizable=no, Scrollbars=yes, copyhistory=no, width=" + width + ", height=" + height + ', top=' + top_position + ', left=' + left_position);    
}