Photographing a star field is the single most difficult test to which a lens can be subjected. Every defect and aberration that a lens has will be revealed in an astrophoto. These include spherical aberrations, chromatic aberrations, coma, astigmatism, curvature of field, and vignetting.
Most lenses perform very poorly when used with the aperture wide open.
Ordinarily we want to shoot our astrophotos with the maximum aperture to record fainter stars and nebulosity in a shorter exposure, but lens aberrations can be so bad as to make this impossible. Almost all lenses will perform much better if stopped down one or two stops. High end lenses made with Fluorite or ED glass from the top manufacturers can however perform very well wide open. A price is paid for this performance though, namely money. They are very expensive.