Hrm. Interesting. Though the tradeoff is also that some dimmer stars won't even show up if you shoot at a smaller aperture, no? I've seen considerably less stars in tests with wider apertures vs. smaller.
Yes, wider apertures = a more dense-looking field of stars. The higher you go, the more the ones that are lesser in intensity are filtered out of the exposure.
You can see by your pictures that f/5.6, and even f/11, can work just fine.
Your aperture for these type of shots should depend on exactly this, as well as some other considerations, such as what kind of exposure you will get on Earth-bound objects in the frame in that amount of time. If you want the most possible light on the Eart-bound objects, then the lower the f stop the better. As a result, you also get a very dense field of stars for these shots.
Vignetting in most f/5.6 LF lenses stops at f/11--this is true for my Rodenstock 55mm, 90mm, and 135mm. However, that is just mechanical vignetting. The wide lenses will have a certain amount of natural vignetting.
You will also have to take care that your film does not shift with humidity during exposure. Many LF astrophotographers convert a 4x5 film holder to a vacuum back. I have heard of folks putting a small piece of double sided tape in the center of the holder to hold the film--but that always seemed a pain to me for various reasons. You may want a cloth to cover the camera so to reduce dewing. There is also the possibility of the lens getting dew on it.
But good luck. More folks should take their cameras out at night--including me.
I'm thinking of getting something like this - Dew-Not
I hadn't really thought of protecting the film plane. Perhaps if I used the dew not on the film area and the lens.
Also, I am not sure I can rent a vacuum film back anywhere. I'm lucky I think that a place has LF gear to rent here in Los Angeles.
my Schneidr Super Angulon 75/5.6 is tack sharp at f11, never shot it more open than that.