This is a great telephoto lens and star testing shows a good behaviour even near full aperture. 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 even at f/3.5, and that for very good puntiformity f/4.5 is best.
The focus test shows that reaching a perfect focus is a difficult task: only after many trials a good focus was reached. Maybe a more demultiplicated foculs control would have helped.
Thanks to Alessandro Gambaro for testing together his objective.