Triple Range Imager and POLarimeter (TRIPOL)-a compact and economical optical imaging polarimeter for small telescopes

We report the design concept and performance of a compact, lightweight and economical imaging polarimeter, the Triple Range Imager and POLarimeter (TRIPOL), capable of simultaneous optical imagery and polarimetry. TRIPOL splits the beam in wavelengths from 400 to 830 nm into g′-, r′-and i′-bands with two dichroic mirrors, and measures polarization with an achromatic half-waveplate and a wire grid polarizer. The simultaneity makes TRIPOL a useful tool for small telescopes for the photometry and polarimetry of time variable and wavelength dependent phenomena. TRIPOL is designed for a Cassegrain telescope with an aperture of ∼1 m. This paper presents the engineering considerations of TRIPOL and compares the expected with observed performance. Using the Lulin 1-m telescope and 100 seconds of integration, the limiting magnitudes are g′ ∼ 19.0 mag, r′ ∼ 18.5 mag and i′ ∼ 18.0 mag with a signal-to-noise ratio of 10, in agreement with design expectation. The instrumental polarization is measured to be ∼ 0.3% in the three bands. Two applications, one to the star-forming cloud IC 5146 and the other to the young variable GM Cep, are presented as demonstrations.