I’m fascinated by these markers that Curiosity has strategically placed all over its topside. I assume they’re positional reference points for the cameras, but I can’t find any solid information about them. Are they meant to help stitch together panoramas? The ratio of the rings goes 1 : 0.6 : 0.45.
They look a lot like registration marks used in printing. In printing they are used to line up overlapping colors (say cyan, magenta, yellow, and black printing plates) into one image.
I’d venture to say the rover uses these to line up exposures taken at the same time from the same origin using various exposure methods like IR, thermal, etc.