function popup_window(loc, w, h)
{
    var win;
    var opt;
    if (w > 1024) w = 1024;
    if (h >  768) h =  768;
    opt  = 'width=' + w + ',height=' + h;
    opt += ',toolbar=0,status=0,location=0,menubar=0';
    opt += ',resizable=1,scrollbars=0';
    win = window.open(loc,"popup", opt);
    if (win.opener == null) win.opener = window;
    win.moveTo((screen.width - w)/2, (screen.height - h)/2);
    win.focus();
}