Polarimetric Imager for Comets, PICO

Abstract

We constructed an imaging polarimeter, PICO, that was optimized for comets and/or cometary objects. PICO has a field-of-view of 5 0 10 0 and employs a fixed Wollaston prism as an analyzer. In the case of polarimeters using a double-beam prism, such as a Wollaston prism, lateral chromatic aberrations originating from the prism often degrade the image quality and measurement accuracy. However, PICO suffers little lateral chromatic aberration due to compensation provided by newly designed lens systems that generate opposite lateral colors. Owing to this design, PICO can obtain highly resolved and very accurate polarization images throughout the FOV (field-of-view). The total accuracy is estimated to be ∆P ' OE.P =85/ 2 + 0:3 2 1=2 % in polarization degree and Â Ä 3 ı in position angle; these values are satisfactory for cometary studies.

Inversion Angle of Phase-polarization Curve of Near-Earth Asteroid (3200) Phaethon

“…The polarimetric survey of Phaethon was performed for 13 consecutive nights from UT 2017 December 9 to December 21 using the Polarimetric Imager for Comets (PICO; Ikeda et al 2007) mounted on the 50-cm Telescope for Public Outreach 1 at Mitaka Campus of National Astronomical Observatory of Japan. We used the standard Johnson-Cousins R C -band filter (its bandpass is 483-799 nm; Bessell 2005) for all observations of Phaethon.…”

“…All selected PICO data were reduced using the standard reduction procedure for imaging observations of a point-source object (dark subtraction, flat-fielding, aperture photometry with the AP P HOT package) described in Ikeda et al (2007) with custom PyRAF script that uses IRAF 2 via Python developed by our group. We also applied a moving-circular aperture to photometry of Phaethon with an elongated shape on the images taken under sidereal tracking on 2017 December 17 (developed by Yoshida & Terai 2017).…”

Imaging polarimetry of Comet 9P/Tempel before and after the Deep Impact

“…The imaging polarimetry of Comet 9P/Tempel was performed using the Lulin One-meter Telescope (LOT) equipped with the Polarimetric Imager for COmets (PICO) developed by Japanese group (Ikeda et al, 2007). Observations were carried out from July 1 to July 10, 2005, covering the DI event.…”

“…As the instrument can take two completely linearly polarized images simultaneously with their polarization angles perpendicular to each other, it is easy to correct the variations caused by the change in sky conditions, altitude of the object, etc. Therefore the PICO measurements remain robust for a change of sky conditions (for more detail, see Ikeda et al, 2007).…”

“…All observational data have been reduced using the standard reduction procedure (Ikeda et al, 2007), except for the estimation of sky background levels. The sky background level was evaluated for each image frame assuming the surface brightness distribution follows a ρ −1 relation where ρ is the projected distance from the optical center of the cometary coma.…”