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.