Issue |
A&A
Volume 481, Number 3, April III 2008
|
|
---|---|---|
Page(s) | 913 - 918 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361:20078302 | |
Published online | 18 February 2008 |
Research Note
Optical spectropolarimetry with incomplete data sets
Department of Astronomy and McDonald Observatory, The University of Texas at Austin, 1 University Station, C1400, Austin, Texas 78712-0259, USA e-mail: jrm@astro.as.utexas.edu
Received:
17
July
2007
Accepted:
29
January
2008
Context. Linear spectropolarimetry is a “photon-hungry” observing
technique, requiring a specific sequence of observations to determine
the Stokes Q and U parameters. For dual-beam spectropolarimeters,
the Q and U Stokes parameters can be ideally determined using
observations at retarder plate positions. The additional
polarization signal introduced by instrumental effects requires the
redundancy of
observations to correct for these effects and to
accurately measure the linear polarization of astronomical objects.
Aims. We wish to determine if the “instrumental signature corrections”
for the Stokes Q and U parameters, and
, are identical for observations with dual-beam
spectropolarimeters. For instances when observations were only
acquired at
retarder plate angles, we wish to determine if the
complete measurement of one Stokes parameter and the associated
instrumental signature correction can be used to determine the other
Stokes parameter.
Methods. We constructed analytical and Monte Carlo models of
a general dual-beam spectropolarimeter to study the factors affecting
the assumption and the uncertainty
thereon. We compared these models with VLT FORS1 linear
spectropolarimetry observations.
Results. We find that, in general,
, with the variance around zero
(
) being directly related to the
signal-to-noise ratio of the observations. Observations of a
polarized standard star, observed under identical instrumental
conditions over the period of 2002-2007, show that the assumption of
is generally true over a long period,
although the absolute values of
and
vary
between observational epochs. While the variance of
is not dependent on the polarization
angle, significant deviations from
arise
when
.
Conclusions. Incomplete VLT FORS1 spectropolarimetry
datasets, for which observations at only retarder plate position
angles have been acquired, can be analyzed under the assumption that
. The uncertainty associated with
this assumption is directly related to the signal-to-noise ratio of
the observations. This property of the analysis of
spectropolarimetry, with dual beam spectropolarimeters, can be used
to test for the presence of artifacts affecting individual
observations and to assess the quality of the data reduction, when
observations at all four retarder plate angles have been acquired.
Key words: instrumentation: polarimeters / polarization
© ESO, 2008
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