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We present an in-depth characterization of the polarimetric channel of theLarge-Angle Spectrometric COronagraph/LASCO-C3 onboard SOHO. The polarimetricanalysis of the white-light images makes use of polarized sequences composed ofthree images obtained through three polarizers oriented at +60$^\circ$,0$^\circ$, and -60$^\circ$, complemented by a neighboring unpolarized image.However, the degradation of the 0$^\circ$ polarizer noticed in 1999 compelledus to reconstruct the corresponding images from the other ones thereafter. Theanalysis closely follows the method developed for LASCO-C2 (Lamy, et al. SolarPhysics 295, 89, 2020 and arXiv:2001.05925) and implements the formalism ofMueller, albeit with additional difficulties notably the presence of anon-axially symmetric component of stray light. Critical corrections werederived from a SOHO roll sequence and from consistency criteria (e.g., thetangential direction of polarization). The quasi-uninterruptedphotopolarimetric analysis of the outer corona over two complete Solar Cycles23 and 24 was successfully achieved and our final results encompass thecharacterization of its polarization, of its polarized radiance, of thetwo-dimensional electron density, and of the K-corona. Comparison between theC3 and C2 results where their field of view overlaps shows an overallagreement. The C3 results are further in agreement with those of eclipses andradio ranging measurements to an elongation of about 10 solar radii but tend todiverge further out. Whereas the coronal polarization out to 20 solar radii isstill highly correlated with the temporal variation of the total magneticfield, this divergence probably results from the increasing polarization of theF-corona.

Coronal Photopolarimetry with the LASCO-C3 Coronagraph over 24 Years  [1996-2019] -- Application to the K/F Separation and to the Determination of  the Electron Density