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The upgrade of a high dispersion spectro-polarimeter, VESPolA: new circular polarimetry mode and extremely high resolution mode

Arasaki, Takayuki; Ikeda, Yuji; Nakamichi, Akika; Kawakita, Hideyo
2014, SPIE
■ アブストラクト
VESPolA is a high dispersion spectro-polarimeter, mounted on the 1.3m Araki telescope at Koyama Astronomical Observatory of Kyoto Sangyo University. It employs an echelle type spectrograph and a non-ripple super-achromatic half wave plate, resulting in the highly polarimetric accuracy of δP < 0.1% with the spectral resolution of R = 8,000 and a wide wavelength coverage of δλ = 150nm - 300nm in a single exposure. It is specialized for the 1-2 m class small telescope with Cassegrain focus. Because VESPolA can observed a dozen of polarization line profiles in the targets simultaneously, we can obtain complementary information on both the geometry and the velocity fields of the circumstellar envelope of variable stars, such as WR stars, Be stars, Herbig Ae/Be stars, T Tau stars, and so on. VESPolA has achieved the high accuracy of δP {(0.05)2 + (0.09)2P2}1/2% with spectral resolution of R 8,000 for liner polarization. Recently, a new scientific program, search for magnetic pole-shift of stars (Nakamichi et al. 20129), was accepted at the Koyama Astronomical Observatory. Therefore, we plan to add a new modes, a high resolution mode (R = 20,000) and a circular polarimetry mode for this study, to VESPolA. We employ an alumina (Al2O3) slit as the new slit for the high resolution mode to minimize the instrumental polarization generated at the edge of the slit. However, because the slit width should be very narrow (= 35μm), it is technically difficult to realize the smooth slit without burr or chipping. We are trying the various processing methods, machining, etching, laser cutting, and so on. The circular polarimetry mode can be realized with a new non-ripple super-achromatic quarter wave plate and the high precision rotary unit, which is located just below the existing the half wave plate unit. We will finish the update within 2015 and start the science observations.