Great shots and good info guys! Keep up the good work damark913!
Sorry for the newbie question. Why f/4? all info I am reading guides me to f 2.8...
Because the lens is exhibiting coma at the edges. Look at how the stars look like "little birds flying" at the edges of the second image. If you stop the lens down, you reduce the coma. You also reduce vignetting at the edges that occurs wide open. Sometimes you have to stop down a few stops to get rid of coma altogether, hence my suggestion to stop down to f/5.6. Of course, then you need to either increase the exposure time or the ISO. Because the D600 has a really clean sensor, he could raise the ISO up a bit, but because he's tracking and has a wide angle lens, he can increase the exposure time, although he will have to composite the foreground in.
Of course, you should always shoot at the maximum aperture you can. The optical quality of the lens will dictate how much you can do that. It's always a trade-off, and balance, of options available to you. Samyang (Rokinon) is about to release a pair of lenses aimed at astrophotographers than can shoot wide open and be sharp to the edges.