Design trade-off and proof of concept for LOUPE, the Lunar Observatory for Unresolved Polarimetry of Earth

LOUPE: observing Earth from the Moon to prepare for detecting life on Earth-like exoplanets

LOUPE, the Lunar Observatory for Unresolved Polarimetry of the Earth, is a small, robust spectro-polarimeter for observing the Earth as an exoplanet. Detecting Earth-like planets in stellar habitable zones is one of the key challenges of modern exoplanetary science. Characterizing such planets and searching for traces of life requires the direct detection of their signals. LOUPE provides unique spectral flux and polarization data of sunlight reflected by Earth, the only planet known to harbour life. These data will be used to test numerical codes to predict signals of Earth-like exoplanets, to test algorithms that retrieve planet properties, and to fine-tune the design and observational strategies of future space observatories. From the Moon, LOUPE will continuously see the entire Earth, enabling it to monitor the signal changes due to the planet's daily rotation, weather patterns and seasons, across all phase angles. Here, we present both the science case and the technology behind LOUPE's instrumental and mission design. This article is part of a discussion meeting issue 'Astronomy from the Moon: the next decades'.

Spectral-temporal hybrid modulation for channeled spectropolarimetry

Channeled spectropolarimeters (CSPs) are capable of estimating spectrally resolved Stokes parameters from a single modulated spectrum. However, channel crosstalk and subsequent spectral resolution loss reduce the reconstruction accuracy and limit the systems' scope of application. In this paper, we propose a spectral-temporal modulation strategy with the aim of extending channel bandwidth and improving reconstruction accuracy by leveraging the hybrid carriers and allocating channels in the two-dimensional Fourier domain that yield optimal performance. The scheme enables spectral bandwidth and temporal bandwidth to be traded off, and provides flexibility in selecting demodulation strategies based on the features of the input. We present an in-depth comparison of different systems' performances in various input features under the presence of noise. Simulation results show that the hybrid-modulation strategy offers the best comprehensive performance as compared to the conventional CSP and dual-scan techniques.

Low crosstalk polarization-difference channeled imaging spectropolarimeter using double-Wollaston prism

A polarization-difference channeled imaging spectropolarimeter (PDCISP) using double-Wollaston prism is presented. It enables simultaneous acquisition of a set of three-channel interferograms corresponding to orthogonal polarization modulation. This brings a large range expanding of optical path difference for useful channels, and the major limitation of channeled spectropolarimetry (CSP), namely the channel crosstalk, can be greatly suppressed by using interferogram difference processing. As a result, full resolution intensity spectrum, as well as high-resolution polarimetric signatures, can be obtained with fewer reconstruction errors, compared to conventional CSP-based systems. The PDCISP is insensitive to alignment errors of retarders and maintains the snapshot feature (1D spatial imaging). The effectiveness of the proposed method is demonstrated by the simulation results.

Reduction of the effects of angle errors for a channeled spectropolarimeter

Angle errors of high-order retarders will decrease the accuracy of a channeled spectropolarimeter. This paper presents an easily implemented and widely applicable method for reducing the effects of the angle errors. First, we theoretically derive a modified reconstruction model to express and analyze the effects of the angle errors. Based on the modified reconstruction model and current reference beam calibration technique, we put forward the modified reference beam calibration technique to reduce the effects of the angle errors. This method can calculate the angle errors by employing the amplitude terms, which have been ignored in the results of the current reference beam calibration. The effectiveness of the presented method is verified by numerical simulations, which show that the demodulated deviations of polarization parameters have been reduced by one order of magnitude. Experiments are further implemented to validate the proposed method. The convenience and wide applicability of the presented method make it suitable for regular correction of the instrument, especially for the case on track.