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.