Figure 1. Context image of the Mastcam-Z primary and secondary calibration targets, taken by the left eye of Mastcam-Z (L0) on sol 23 at 26 mm focal length (Image-ID: ZL0_0023_0668982043_910IOF_N0030770ZCAM03003_0260LMA01, sequence ZCAM03003). In the upper center, the SuperCam calibration target is also visible.
The principal device, known as “primary target”, is bolted to the top of the Rover Pyro Firing Assembly (RPFA) and consists of eight round color and grayscale ceramic patches, four concentric grayscale rings and a central shadow post fixed on a gold-plated aluminum frame. The circular patches, mounted in the periphery of the primary target, are four in colors (here referred to as green, yellow, blue, red) and four in grayscales (black, dark gray, light gray, white). Eight hollow-cylindrical magnets are mounted underneath the round patches. These Sm2Co17 magnets are strongly magnetized along the axis of symmetry. They attract even weakly magnetic (in the sense of having significant magnetic susceptibility) martian dust grains on the external annular portion of the patches, while actively repelling such grains from the central circular part, leaving it relatively clean from dust. The grayscale rings placed in the center of the target are the same colors and materials as the peripheral grayscale patches. From the innermost to the outermost, the colors of the rings are light gray, black, white, and dark gray. The shadow post (or gnomon) is placed at the center of the rings and painted with an IR-black paint. The base of the primary target is made from aluminum with silver and gold anodization and fits inside a square with a side of 98 mm. The “secondary target” is fixed on the vertical side of the RPFA just below the primary target, in direct sight of Mastcam-Z, and contains two rows of seven square tiles each mounted on an aluminum frame. One row is situated parallel to the plane of the rover deck, while the other row is situated perpendicularly. Both rows have the same color tiles, with the colors being the same as the round patches of the primary target except for the yellow. From left to right, as seen from Mastcam-Z, they are black, dark gray, light gray, white, red, green, and blue. The horizontal length of the secondary target is 80 mm. For a complete technical description of the calibration targets and their heritage from previous Mars rovers, see Kinch et al. (2020).
In this work we adopt a simple nomenclature scheme for the different patches and rings according to their positions, which is illustrated in Figure 2. In the primary target, the inner circular portions of the eight round patches are named “central spots” or “clean spots”, while the external annular portions, which lie over the magnets, are called “magnet rings”, not to be confused with the four concentric grayscale rings at the center of the primary target. The name “clean spots” is a consequence of the action of the magnet rings, which capture the magnetic fraction of the airborne dust. For the secondary target, the square patches are named “secondary horizontal tiles” or “secondary vertical tiles”, depending on their orientation.