Table 1. Summary of the total
number of images of the Calibration Targets taken by Mastcam-Z in each
filter within the first 350 sols on Mars. For each filter, the
corresponding spectral range is reported.
These images were captured by Mastcam-Z at several illumination
geometries relative to the cal-targets. The illumination geometry of an
observation is described by three angles. We define these three angles
as incidence , emission and azimuth . The incidence
angle (\(0\leq i\leq 90\)) is measured between the zenith of the
target and the Sun-target direction, the emission angle
(\(0\leq e\leq 90\)) is measured between the zenith of the target and
the detector-target direction, while the azimuth angle
(\(0\leq\text{Az}\leq 180\) for hemispheric symmetry) is measured on
the plane of the target and is the angle between the planes of incidence
and emission. The angles \(i\) and \(e\) are measured from the zenith.
The emission angle is fixed by the geometry of the rover with the
primary and secondary targets seen by Mastcam-Z under emission angles of
approximately 58 and 54, respectively. The overall distributions
of the incidence and azimuth angles of the cal-target images are
reported in Figure 3. A total of 197 cal-target sequences (which include
images in only L0/R0 or all filters) were acquired by Mastcam-Z over the
first 350 sols, each of which can be identified by its incidence and
azimuth angles. The left plot in Figure 3 shows the combinations of
incidence and azimuth angles of those 197 sequences. Summing the data
points along the two axes of the plot yields the two histograms of
azimuth and incidence in Figure 3 on the right. Given the changing
orientations of Perseverance, the azimuth angles are evenly distributed,
though with a larger frequency in the 5-10 interval, which
corresponds to the Sun being approximately behind Mastcam-Z during the
observation (Sun in front of the rover), and a much lower frequency of
observations at opposite azimuth. In addition, most observations were
made with the Sun relatively high above the horizon, with 88% of images
taken at incidence angles lower than 30 and a peak around
10-15.
As studied by Buz et al. (2019) and by Kinch et al. (2020)
in the pre-flight tests, at the typical emission angle of 58 all the
color and grayscale materials of the cal-targets (though to a lesser
extent for the white patches) show a strong forward scattering peak with
incidence angles \(>45\). The smaller number of observations on Mars
with the Sun opposite to the camera (high azimuth) reduced the impact of
the forward scattering, which in some cases (\(i\geq 50\)) was quite
significant. In addition, at higher incidence angles the diffuse light
acquired importance within the dusty atmosphere, reducing the contrast
between the sunlit and shadowed regions. Early martian morning or late
afternoon observations proved challenging for estimating the solar
irradiance as part of the reflectance calibration, and this will serve
as a starting point for improvements in our data processing in the
future. The best estimates of the irradiance were obtained when the Sun
was high in the sky, with a smaller atmospheric path length and
negligible forward or backward scattering. Mastcam-Z acquired most
cal-target images at incidence angles lower than 30. However, since
the diffuse light had a major impact at low Sun, this geometry was
particularly useful to monitor the dust suspended in the atmosphere.
This was done by comparing the radiance from the regions in the gnomon
shadow on the grayscale rings to those in the direct sunlight (see sect.
4.3.2).