Stray light caused by scattering on optical surfaces and in the Earth's
atmosphere degrades the spatial resolution of observations. We study the
contribution of stray light to the two channels of POLIS. We test the
performance of different methods of stray-light correction and spatial
deconvolution to improve the spatial resolution post-facto. We model the stray
light as having two components: a spectrally dispersed component and a
component of parasitic light caused by scattering inside the spectrograph. We
use several measurements to estimate the two contributions: observations with a
(partly) blocked FOV, a convolution of the FTS spectral atlas, imaging in the
pupil plane, umbral profiles, and spurious polarization signal in telluric
lines. The measurements allow us to estimate the spatial PSF of POLIS and the
main spectrograph of the German VTT. We use the PSF for a deconvolution of both
spectropolarimetric data and investigate the effect on the spectra. The
parasitic contribution can be directly and accurately determined for POLIS,
amounting to about 5%. We estimate a lower limit of about 10% across the full
FOV for the dispersed stray light. In quiet Sun regions, the stray-light level
from the close surroundings (d< 2") of a given spatial point is about 20%. The
stray light reduces to below 2% at a distance of 20" from a lit area for both
POLIS and the main spectrograph. A two-component model of the stray-light
contributions seems to be sufficient for a basic correction of observed
spectra. The instrumental PSF obtained can be used to model the off-limb stray
light, to determine the stray-light contamination accurately for observation
targets with large spatial intensity gradients such as sunspots, and also
allows one to improve the spatial resolution of observations post-facto.Comment: 14 pages, 16 figures, accepted by A&A. Version V2 revised for
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