Detecting polarization of the light reflected from an exoplanet requires extremely high-precision polarimeters and highaccuracy calibration techniques. The polarimetric precision of a few parts per million (ppm), approaching the photon noise, was demonstrated for the Sun and bright distant stars by several groups and instruments. However, the accuracy of absolute polarimetric calibration strongly depends on the polarimeter design and observing conditions, which results in largely unknown systematic errors hindering the exoplanet polarization detection. Here we discuss some of the crucial aspects of exoplanet polarimetric data acquisition, e.g., effects of seeing, sky polarization, telescope polarization, etc. We simulate examples of polarimetric measurements with various levels of random and systematic errors. They demonstrate that sparse measurements (ten or less) and unknown systematic errors can hinder exoplanet signal detection even when the signal is significantly larger than the polarimetric precision. We discuss various approaches which help improve random errors (precision) and mitigate systematic errors (accuracy) caused by various effects. We also discuss the performance of polarimeters with different designs and indicate their strengths and weaknesses in terms of precision and accuracy.
We report the current status of small-telescope activities and the 1.8-m aperture telescope PLANETS project at Haleakala dedicated to planetary and exoplanetary observations. Continuous monitoring is essential to understand the planetary atmospheric phenomena, and therefore, own facilities with even small- and medium sized telescopes and instruments are important. On the summit of Mt. Haleakala, Hawaii, we are operating a 40 cm (T40) and 60 cm (T60) telescopes for measuring faint atmospheric features such as Io torus, Mercury, and so on. It has uniquely provided long-term Io torus activities for more than ten years. T60 is now observing planetary atmospheres in visible and infrared ranges. The polarization imager DIPOL-2 is also installed to measure the weak polarization of exoplanetary light. In addition, we are carrying out a 1.8-m off-axis telescope project PLANETS at Haleakala. This project is managed by the PLANETS Foundation (www.planets.life) is an international collaboration of several institutes from Japan, USA, Germany, Brazil, and France. This off-axis optical system enables very low-stray light contamination and high-contrast in data, i.e., "high dynamic range". It will achieve unrivaled scientific capabilities on coronagraphy and polarimetry, aimed at detecting exoplanet reflected light and tenuous planetary exo-atmospheres in the Solar system. The main mirror is Clearceram ZHS with a diameter of 1850 mm, which is now on the final polishing process. We completed the telescope design and wind analysis of the mechanical support and tracking. The "split-ring" mount is so stiff that it has a first vibration mode above 50 Hz.
We have built a new broad-band polarimeter (DIPOL-2), capable of measuring polarization with the precision at the 10-5
level. Two dichroic beam-splitters are used to split light into three passbands (BVR), simultaneously recorded with three
CCDs. A rotatable superachromatic λ/2 (or λ/4) plate is used as the polarization modulator and a plane parallel calcite
plate as the polarization analyzer. Simple yet effective design with small number of optical elements and moving
mechanical parts makes DIPOL-2 a highly versatile and reliable instrument with negligible instrumental polarization,
very well suitable for observations with remotely controlled telescopes. Current and planned observing programs are
briefly described.
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