Seams can be a problem not only in the dome master frames, as Pete illustrated in previous posts, but also in the projectors themselves. I learned from our resident dome expert, Eric Knisely, that seams can present themselves in the projectors themselves. Constant maintenance is needed to keep these projectors calibrated, and not every planetarium may be keeping up with theirs. Eric let me know how using solid colors can really make these seams stand out. He suggested that noise be added to them to break up the constant color, where even gradients may fall short. I can show in these quick illustrations below the scene that brought this problem to light, pardon the pun. The seams I put in are of my own doing, but I tried to make it reflect what we saw in the dome the best I could. The dome at RENCI uses 4 cameras, so the seam across the top was a cross shape.
The first example is what it would look like had it been with just a single projector.
The second is the same image projected using 4 projectors.
The third is with the noise applied to the sky, breaking up the values to help hide the seams.