A Deconvolution Technique to Correct Deep Images of Galaxies from
  Instrumental Scattered Light
  Deep imaging of the diffuse light emitted by the stellar fine structures and
outer halos around galaxies is now often used to probe their past mass
assembly. Because the extended halos survive longer than the relatively fragile
tidal features, they trace more ancient mergers. We use images reaching surface
brightness limits as low as 28.5-29 mag.arcsec-2 (g-band) to obtain light and
color profiles up to 5-10 effective radii of a sample of nearby early-type
galaxies. They were acquired with MegaCam as part of the CFHT MATLAS large
programme. These profiles may be compared to those produced by simulations of
galaxy formation and evolution, once corrected for instrumental effects. Indeed
they can be heavily contaminated by the scattered light caused by internal
reflections within the instrument. In particular, the nucleus of galaxies
generates artificial flux in the outer halo, which has to be precisely
subtracted. We present a deconvolution technique to remove the artificial halos
that makes use of very large kernels. The technique based on PyOperators is
more time efficient than the model-convolution methods also used for that
purpose. This is especially the case for galaxies with complex structures that
are hard to model. Having a good knowledge of the Point Spread Function (PSF),
including its outer wings, is critical for the method. A database of MegaCam
PSF models corresponding to different seeing conditions and bands was generated
directly from the deep images. It is shown that the difference in the PSFs in
different bands causes artificial changes in the color profiles, in particular
a reddening of the outskirts of galaxies having a bright nucleus. The method is
validated with a set of simulated images and applied to three representative
test cases: NGC 3599, NGC 3489, and NGC 4274, and exhibiting for two of them a
prominent ghost halo. The method successfully removes it.
