The image that the liquid-crystal-panel-based DOLPi takes at “45 degrees” is not strictly that, which is why I state in the paper:
“Bossa Nova’s method is straightforward if laboratory optical-grade components are used. These are very expensive and out of reach for most private enthusiasts. However, I found through experimentation that a welding mask LCP and a polarizer sheet can also give very satisfactory results.”
In reality, the LCP driven half-way acts as a quarter-wave plate, and hence the strict interpretation of the analysis at this level is for circular polarization rather than linear polarization at 45 degrees.
I didn’t want to go into a thorough explanation of polarization optics to keep the project accessible, but based on my experiments, I’m convinced that DOLPi’s “45 degree image” indeed contains a dominant 45 degree component when observing linearly polarized light.
This weekend I decided to build a mechanical filter-wheel-based polarimetric camera to serve as a basis for comparison to the LCP_based DOLPi. This camera is much slower than the LCP-based DOLPi because of the mechanical switching of filters, but it provides the data necessary for complete Stokes imaging (including the fourth Stokes parameter describing circular polarization). The pictures that it produces are of excellent quality!