google maps: recenter a map

See:http://stackoverflow.com/questions/14285251/google-maps-redefine-center-point-fit-bounds-to-specific-area/14285910#14285910

HTML

<script src="https://maps.googleapis.com/maps/api/js?v=3.exp&sensor=false"></script>
    <script>
      var map;
      function initialize() {
        var mapOptions = {
          zoom: 8,
          center: new google.maps.LatLng(-34.397, 150.644),
          mapTypeId: google.maps.MapTypeId.ROADMAP
        };
        map = new google.maps.Map(document.getElementById('map_canvas'),
            mapOptions);new google.maps.Marker({map:map,position:new google.maps.LatLng(-34.397, 150.644)})
            map.panBy(20,-20);
      }

      google.maps.event.addDomListener(window, 'load', initialize);
    </script>
    <div id="map_canvas" style="width:300px;height:300px"></div>
    <div style="width:260px;height:260px;opacity:.5;border-right:40px solid #000;border-top:40px solid #000;position:absolute;top:0;left:0"></div>