A way to deal with the fringe-like pattern in VIMOS-IFU data
  The use of integral field units is now commonplace at all major observatories
offering efficient means of obtaining spectral as well as imaging information
at the same time. IFU instrument designs are complex and spectral images have
typically highly condensed formats, therefore presenting challenges for the IFU
data reduction pipelines. In the case of the VLT VIMOS-IFU, a fringe-like
pattern affecting the spectra well into the optical and blue wavelength regime
as well as artificial intensity variations, require additional reduction steps
beyond standard pipeline processing. In this research note we propose an
empirical method for the removal of the fringe-like pattern in the spectral
domain and the intensity variations in the imaging domain. We also demonstrate
the potential consequences for data analysis if the effects are not corrected.
Here we use the example of deriving stellar velocity, velocity dispersion and
absorption line-strength maps for early-type galaxies. We derive for each
spectrum, reduced by the ESO standard VIMOS pipeline, a correction-spectrum by
using the median of the eight surrounding spectra as a proxy for the
unaffected, underlying spectrum. This method relies on the fact that our
science targets (nearby ETGs) cover the complete FoV of the VIMOS-IFU with
slowly varying spectral properties and that the exact shape of the fringe-like
pattern is nearly independent and highly variable between neighboring spatial
positions. We find that the proposed correction methods for the removal of the
fringe-like pattern and the intensity variations in VIMOS-IFU data-cubes are
suitable to allow for meaningful data analysis in our sample of nearby
early-type galaxies. Since the method relies on the scientific target
properties it is not suitable for general implementation in the pipeline
software for VIMOS.
