Polarimetric Littrow Spectrograph – instrument calibration and first measurements

C. Beck; W. Schmidt; T. Kentischer; D. Elmore

doi:10.1051/0004-6361:20052662

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Volume 437 / No 3 (July III 2005)

A&A, 437 3 (2005) 1159-1167

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Issue		A&A Volume 437, Number 3, July III 2005


Page(s)		1159 - 1167
Section		Astronomical instrumentation
DOI		https://doi.org/10.1051/0004-6361:20052662
Published online		30 June 2005

A&A 437, 1159-1167 (2005)

Polarimetric Littrow Spectrograph – instrument calibration and first measurements

C. Beck¹, W. Schmidt¹, T. Kentischer¹ and D. Elmore²

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: cbeck,wolfgang,tk@kis.uni-freiburg.de
² High Altitude Observatory, National Center for Atmospheric Research, 3450 Mitchell Lane, Boulder, Colorado 80301, USA e-mail: elmore@ucar.edu

Received: 10 January 2005
Accepted: 5 April 2005

Abstract

We present first measurements and the calibration procedure for the Polarimetric Littrow Spectrograph (POLIS) operated at the Vacuum Tower Telescope on Tenerife, together with a brief summary of the technical characteristics of the instrument. In its present configuration, we achieve a polarimetric accuracy of about $3 \times 10^{-3}$ in the visible channel (630 nm) of the instrument. The accuracy is limited by cross talk among the different polarization states. The detection limit for polarized light is about $2 \times 10^{-3}$ for a 7 s exposure. Polarimetric measurements in the blue channel ( $\ion{Ca}{ii}$ H line, 396.7 nm) are strongly limited by the low photon flux. At this wavelength we present Stokes-V maps with a spatial resolution of about 0.5 arcs. The polarimetric quality of any spectropolarimeter is limited by the precision of the instrument calibration. We present a new method for self-calibration that reduces cross talk among the polarization components to 0.1%. This improvement results from a measurement of the retardance of the calibration waveplate with an accuracy of 0.1°. We demonstrate the capability of the simultaneous use of POLIS and the Tenerife Infrared Polarimeter which is integrated in the main spectrograph of the Vacuum Tower Telescope.

Key words: Sun: magnetic fields / instrumentation: polarimeters

© ESO, 2005

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