This is a great zoom lens and star testing shows that a good behaviour can be obtained with a small stopping down. Testing was made on a clear and cold night of December 2010, with automatic tracking of my Kenko NES mount. Exposures and ISO were selected to maintain a constant nominal exposure, except the two higher apertures.
The results show that a good star shape is obtained at f/5.6.
Thanks to Alessandro Gambaro for testing together his objective.