Imaging Spectral Polarimeter for Planetary Observations

The view of planetary surfaces from orbit is commonly attenuated due to scattering by atmospheric particulates which induce a polarization to the scattered light. The planetary surface may be viewable only through narrow spectral ‘windows’ in their atmospheres. One of the techniques used around several planets in our solar system to widen these windows is to take images in a polarization maximizing light visible from the surface and minimizing the atmosphere’s contribution. The polarization changes with observation and sun phase angle and is often limited in applicability. In this study, we are investigating the effects of polarization on spectral passband using a custom comprehensive polarization analysis optical system.

We conducted laboratory and field experiments including observation of the polarization of artificial rainbows and naturally-variable haze present in a field site with a wide range of background distances. Commercial imaging polarimeters were unable to provide the information necessary to complete these studies, so we constructed a custom imaging spectral polarimeter containing five spectral filters of wavelengths 490 through 650 nanometers and one unfiltered broadband channel, a linear polarizer, and an optional broadband quarter waveplate. We use two OEM piezo rotation motors to independently rotate a custom 3D printed filter wheel and the linear polarizer. The images for the data are taken by a commercial CMOS camera and controlled by in-house software for data management and processing.

We report here the results of the initial experiments which include a measurement of the degree of polarization of the artificially created rainbows as a function of spectral bandpass. In addition, we present the spectral polarimetry results of the field site observations and the relevant polarizations and degrees of polarization through a natural haze. We conclude with a description of the future instrument development path toward addressing near infrared atmospheric windows occluded by particle scattering.