I am well aware of the Cosmic Ark problem. The starfield is generated by using the TIA in an unpredictable way (see the warning under HMP0 on page 41
here). The assembly instructions that create this effect are well known, but no one actually understands what causes the effect. I have done plenty of research into this including running a transistor level simulation of the chip which was created from scanning the actual TIA. The simulator did not recreate the effect, which leads me to think that this is either a race condition in the chip where one trace is longer than the other, or that there is an analog effect happening with the transistors that will be very difficult to understand. I am still somewhat looking into this, but I don't expect to find much.
Stella is more high level and they use more hackish methods to implement some features, including this starfield. They only recreate the behavior, they don't try to simulate the low-level effects (search for "cosmic" in
this source file). I wrote Atarihawk to be close to cycle-accurate, and I prefer to only implement changes if they are well understood, instead of doing something because "that's how stella does it." I wish I understood how this starfield works, but until I do I am unlikely to implement it in Atarihawk.
