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

Abstract

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

HO Puppis: Not a Be Star, but a Newly Confirmed IW And-type Star

“…Polarimetric data of HO Pup were well obtained in r and i bands (but it were too faint in the g band), as reported in Table 3. We have also observed a number of unpolarized and polarized standard stars (Schmidt et al 1992) to calibrate the instrumental polarization and angle offset (Sato et al 2019). Based on the observations of these standard stars, we found that the performance of TRIPOL2 was very stable during those nights for both the measurements of polarization percentages (using both polarized and unpolarized standard stars) and the polarization position angles (using the unpolarized standard stars); for more details, see Huang (2019).…”

“…In addition to the photometric and spectroscopic observations, we also took polarimetric data by using the TRIPOL2 (second generation of the Triple Range Imager and Polarimeter) instrument (Sato et al 2019), equipped on the Lulin One-meter Telescope (LOT), in 2018 October. TRIPOL2 can simultaneously take polarization images in Sloan g′, r′, and i′ bands with the half-wave plate rotating to four angles: 0°, 22°.5, 45°, and 67°.5.…”