No, it was done with a circular polariser in front of the lens, I don’t have a large enough linear polariser for those lenses.
Anyway, one image is taken as the zero degree image, then the pol is rotated through 45 deg, then another 45 deg, then one more. Giving 0, 45, 90 and 135 degree images. Even that’s not going to be very accurate!
The polarisation functions in IRIS are used to produce the various maps.
Oddly enough on the second night, you could easily see the difference in the various frames due to the sky pol on the camera LCD display. IRIS offers several pol functions but I don’t know how it’s actually determining the various vectors to produce the images.
Everybody seems to get obsessed taking images I just wanted to try something different and see if it would work! The fact that the Rayleigh scattering/polarisation is perpendicular to the solar direction would seem to suggest it was successful after a fashion… but how accurate, who knows! That’s why I say the stronger pol nearer the nucleus could still be an artifact.
Perhaps someone else can try it…?, the comet is still around!