This is a standard quality zoom lens; star testing shows that a fairly decent behaviour can be obtained with two 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 decent star shape is obtained at f/5.6, even if a residual radial elongation is still present.
Thanks to Alessandro Gambaro for testing together his objective.