I have the Canon 17-40mm f/4L and it works very nicely for landscapes. I have not tried any star photography but I have read that wider apertures are better and of course a 2.8 or 1.4 lens will be easier to use in low light. With the 1DX, however, I can shoot with very high ISO and use the 17-40mm in many instances. I bought it for landscapes where it performs very well compared to the 16-35mm f/2.8L for a lot less money. Here is an example of a recent image with the 17-40mm f/4L: