function openPopup(url, width, height) {
    /* open and center a popup window */
    var sw = screen.width;
    var sh = screen.height;
    var left = sw / 2 - width / 2;
    var top = sh / 2 - height / 2;
    var win = window.open(url, 'someWindow', 
                'width=' + width + ',height=' + height + ',left=' + left + ',top=' + top);
    return win;
};