function openCentered (url, width, height, windowName, featureString) {

  if (!windowName)

    windowName = '';

  if (!featureString)

    featureString = '';

  else

    featureString = ',' + featureString;

  var x = Math.round((screen.availWidth - width) / 2);

  var y = Math.round((screen.availHeight - height) / 2);

  featureString = 'left=' + x + ',top=' + y + ',width=' + width 

+ ',height=' + height + featureString;

  open (url, windowName, featureString);

}