Coronal Electron Densities Derived with Images Acquired during the 2017 August 21 Total Solar Eclipse

The total solar eclipse of 2017 August 21 was observed with a digital single lens reflex (DSLR) camera equipped with a linear polarizing filter. A method was developed to combine images acquired with 15 different exposure times (from 1/4000 s to 4 s), identifying in each pixel the best interval of detector linearity. The resulting mosaic image of the solar corona extends up to more than 5 solar radii, with a projected pixel size of 3.7 arcsec/pixel and an effective image resolution of 102, as determined with visible alpha-Leo and nu-Leo stars. Image analysis shows that in the inner corona the intensity gradients are so steep that nearby pixels show a relative intensity difference of up to similar to 10%; this implies that care must be taken when analyzing single exposures acquired with polarization cameras. Images acquired with two different orientations of the polarizer have been analyzed to derive the degree of linear polarization and the polarized brightness pB in the solar corona. After intercalibration with pB measurements by the K-Cor instrument on Mauna Loa Solar Observatory (MLSO), the data analysis provided the 2D coronal electron density distribution from 1.1 up to similar to 3 solar radii. The absolute radiometric calibration was also performed with the full Sun image and with magnitudes of visible stars. The resulting absolute calibrations show a disagreement by a factor of similar to 2 with respect to MLSO; interestingly, this is the same disagreement recently found with eclipse predictions provided by MHD numerical simulations.