A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

Free access article

Issue		A&A Volume 443, Number 3, December I 2005


Page(s)		1047 - 1053
Section		The Sun
DOI		http://dx.doi.org/10.1051/0004-6361:20052935

A&A 443, 1047-1053 (2005)
DOI: 10.1051/0004-6361:20052935

A polarization model for the German Vacuum Tower Telescope from in situ and laboratory measurements

C. Beck¹, R. Schlichenmaier¹, M. Collados², L. Bellot Rubio^{1, 3} and T. Kentischer¹

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany
    e-mail: [cbeck;schliche;lbellot;tk]@kis.uni-freiburg.de
² Instituto de Astrofísica de Canarias, via Láctea, La Laguna, Tenerife, Spain
    e-mail: mcv@ll.iac.es
³ Instituto de Astrofísica de Andalucía, Apdo. 3004, 18080 Granada, Spain
    e-mail: lbellot@iaa.es

(Received 25 February 2005 / Accepted 21 July 2005)

Abstract
It is essential to properly calibrate the polarimetric properties of telescopes, if one wants to take advantage of the capabilities of high precision spectro-polarimeters. We have constructed a model for the German Vacuum Tower Telescope (VTT) that describes its time-dependent polarization properties. Since the coelostat of the telescope changes the polarization state of the light by introducing cross talk among different polarization states, such a model is necessary to correct the measurements, in order to retrieve the true polarization as emitted from the Sun. The telescope model is quantified by a time-dependent Mueller matrix that depends on the geometry of the light beam through the telescope, and on material properties: the refractive indices of the coelostat mirrors, and the birefringence of the entrance window to the vacuum tube. These material properties were determined experimentally in-situ by feeding the telescope with known states of polarization (including unpolarized light) and by measuring its response, and from measurements of an aluminum-coated sample in the laboratory. Accuracy can in our case be determined only for the combination of telescope and spectro-polarimeter used; for the instrument POLIS at the VTT, we estimate an accuracy of $\pm$ 4- $5\times 10^{-3}$ for the cross talk correction coefficients.

Key words: instrumentation: polarimeters -- sun: magnetic fields

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access. An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
You can define your own OpenURL resolver with your EDPS Account. In this case your choice will be given priority over that of your library.
You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.