Bibcode
Beck, C.; Rezaei, R.; Fabbian, D.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 535, id.A129
Advertised on:
11
2011
Revista
Número de citas
34
Número de citas referidas
31
Descripción
Context. Stray light caused by scattering on optical surfaces and in the
Earth's atmosphere degrades the spatial resolution of observations.
Whereas post-facto reconstruction techniques are common for 2D imaging
and spectroscopy, similar options for slit-spectrograph data are rarely
applied. Aims: We study the contribution of stray light to the
two channels of the POlarimetric LIttrow Spectrograph (POLIS) at 396 nm
and 630 nm as an example of a slit-spectrograph instrument. We test the
performance of different methods of stray-light correction and spatial
deconvolution to improve the spatial resolution post-facto.
Methods: We model the stray light as having two components: a spectrally
dispersed component and a "parasitic" component of spectrally
undispersed light caused by scattering inside the spectrograph. We used
several measurements to estimate the two contributions: a) observations
with a (partly) blocked field of view (FOV); b) a convolution of the FTS
spectral atlas; c) imaging of the spider mounting in the pupil plane; d)
umbral profiles; and e) spurious polarization signal in telluric
spectral lines. The measurements with a partly blocked FOV in the focal
plane allowed us to estimate the spatial point spread function (PSF) of
POLIS and the main spectrograph of the German Vacuum Tower Telescope
(VTT). We then used the obtained PSF for a deconvolution of both
spectroscopic and spectropolarimetric data and investigated the effect
on the spectra. Results: The parasitic contribution can be
directly and accurately determined for POLIS, amounting to about 5%
(0.3%) of the (continuum) intensity at 396 nm (630 nm). The spectrally
dispersed stray light is less accessible because of its many
contributing sources. We estimate a lower limit of about 10% across the
full FOV for the dispersed stray light from umbral profiles. 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. The spatial deconvolution using the PSF obtained
improves the spatial resolution and increases the contrast, with a minor
amplification of noise. Conclusions: 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 to improve the spatial resolution
of observations post-facto.
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