On the Mars Curiosity rover, there are black and white circular patterns or marks spaced around the rover, as seen here marked in lime green:
What are these?
If I had to guess, I'd wager they're points of reference, like to determine alignment of the rover and its parts, and/or the relative size/position/distance/focus of things, whether on-rover or in the surrounding environment. They also look very similar to patterns found on "focus cards" used for cameras.