Figure 7. Appearance of the
calibration targets in 4 Mastcam-Z color images taken by the left eye on
(a) sol 23, (b) sol 130, (c) sol 237 and (d) sol 346. Image (a) was
acquired at 63 mm zoom and had sequence number ZCAM03003, while (b), (c)
and (d) at 48 mm and had sequence number ZCAM03014. Image-IDs: (a)
ZL0_0023_0668982767_910IOF_N0030770ZCAM03003_0630LMA01, (b)
ZL0_0130_0678477333_428IOF_N0042222ZCAM03014_048085A03, (c)
ZL0_0237_0687977979_443IOF_N0072050ZCAM03014_048085A02, (d)
ZL0_0346_0697653263_269IOF_N0092982ZCAM03014_048085A02. The four
images have the same color thresholds (Red: [0.014054-0.990546],
Green: [0.013349-0.928501], Blue: [0.014054-0.959881]) and the
gamma set to 1.5.
Since the ROIs that were normally used for IOF calibration were the
eight clean spots, it is interesting to show some statistics of the
number of pixels for the corresponding eight regions at three zoom
settings: the two extremes (26 mm and 110 mm) and the most employed zoom
setting (48 mm). The 26 mm zoom was used to image the cal-targets only
at the beginning of the mission, between sols 9 and 26, in a total of 54
frames. The average number of pixels over the eight regions and the 54
frames was approximately 15. For the 110 mm zoom, the average on the
values obtained on Mars (42 frames) was 396 pixels. Kinch et al.(2020) considered the shapes of the clean spots as ellipses when seen
from Mastcam-Z, retrieving their major and minor axes respectively of 10
by 5 pixels for the 26 mm zoom and 40 by 20 pixels for the 110 mm zoom.
These numbers give areas of 40 and 630 pixels, respectively. The values
that we obtained were lower than these estimates, likely because their
prediction was computed over the entire surface of the clean spots. The
algorithm that selected those regions always picked a smaller central
portion of the clean spots, leaving a reasonable margin from the
boundary of the magnet rings in order not to select dusty pixels that
could systematically affect our measurements. However, 93% of the total
number of cal-target images in the first 350 sols of the mission were
captured with a 48 mm focal length. For this setting, the average number
of pixels was 67.
Solar Irradiance Time
Series
The slopes of the one-term linear fits used for IOF calibration are
indicators of the instantaneous solar irradiance. Because this could be
affected by multiple factors, such as Sun-Mars distance, atmospheric
conditions, dust and light diffusion, monitoring the slope values is an
efficient way to evaluate indirectly the martian environment and its
variations in time. In addition, the illumination geometry could
influence the slope extracted from the fit, especially when the Sun is
low on the horizon. Therefore, we distinguished between observations
with incidence angle smaller and greater than 30. The plot in Figure
8a shows the temporal evolution of the slopes for the 12 narrow-band
filters. All filters followed a smooth curve with a higher dispersion at
shorter wavelengths, forming a shallow depression with the minimum on
the martian aphelion, which was expected due to the larger distance from
the Sun. The maximum will occur at the perihelion (sol 475). Notably,
around sol 315 a major wind event left new particles on the cal-targets
and raised a significant amount of dust in the atmosphere, causing a
sudden drop in the irradiance. Since then, the irradiance kept
increasing at levels lower than those before the event.