Bibcode
Beck, C.; Schlichenmaier, R.; Collados, M.; Bellot Rubio, L.; Kentischer, T.
Bibliographical reference
Astronomy and Astrophysics, Volume 443, Issue 3, December I 2005, pp.1047-1053
Advertised on:
12
2005
Journal
Citations
73
Refereed citations
64
Description
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 ±4-5× 10-3 for the cross
talk correction coefficients.