The recent threads on Star Castle for the Atari 2600 have been rather annoying, so I decided to see how difficult it would be to write my own version.
It turns out not to be too difficult - I was able to write a basic kernel with rotating shields in a single weekend. I just went for a simple implementation using extra RAM (FA bank-switching) instead of pre-calculating all the ring data. The advantage of this approach is that the rings can be made to spin at different speeds.
It isn't quite as advanced as the kernel that Supercat posted, but it avoids some flicker by using only a single sprite to draw the rings. I also used the size calculations that Thomas posted previously. Note that it looks best on a real TV, or using the phosphor (ALT-p) mode in Stella.
I'm not sure if I'll take this further or not. I have posted the binary and source in case anyone else is interested.
Let me know what you think?