Ground-based ultraviolet polarimetry for imaging the Babinet polarization neutral point

Monitoring the polarization signature of the sky is valuable for navigation, meteorology, and remote sensing. However, the neutral points—Babinet (above the Sun), Brewster (below the Sun), Arago (above the anti-Sun), and the Fourth (below the anti-Sun)—remain underutilized as meteorological tools. The neutral points are formed through multiple scattering, making them useful markers for atmospheric turbidity, but further research is needed to understand their positional changes under varying conditions. This study presents the development of the Ultraviolet Linear Stokes Imaging Polarimeter (ULTRASIP), which operates at a center wavelength of 355 nm with a 10 nm bandpass and an instantaneous field of view of 7.2 arcseconds/pixel. A novel estimation technique to determine the position of the neutral point from linear Stokes images is applied to 10 h of observations. The true position of the neutral point is unknown, so the Babinet neutral point positions are reported with linear regression uncertainties. The position of the Babinet neutral point was found with azimuth uncertainties ranging from 2.52 to 21.96 arcseconds and altitude uncertainties ranging from 2.16 to 5.40 arcseconds. To the authors’ knowledge, these are the first images of the Babinet neutral point in an ultraviolet waveband. The development of ULTRASIP and neutral point position estimation technique will support future correlation studies between neutral point positions and atmospheric turbidity.