MATLAS, a complete optical sample to probe the stellar populations on the outskirts of galaxies: overview and instrumental challenges

The MATLAS project (Mass Assembly of early-Type GaLAxies with their fine Structures) has now turned into an extensive collection of early-type and late-type galaxies. It is made of more than 250 early-type galaxies from the ATLAS^3D survey and 150 spirals, many of which from the NGVS. Unique by its size, the MATLAS volume limited sample is effective to characterize the external parts of massive nearby galaxies and constraints models used in numerical simulations of mergers, the mass assembly of galaxies leaving various imprints in their surroundings, such as shells, streams, tails, tidal dwarf galaxies. Detecting those extremely faint features is instrumental in understanding further the mechanisms driving the mass assembly. The MATLAS sample is made exclusively of ultra deep optical images obtained with MegaCam on the CFHT, leading to colour profiles at large galactic radii, 5 - 10 effective radii. Following a general presentation of MATLAS and its main results, I will report on advances in deep low surface brightness photometry using the wide-field instrument MegaCam. Focusing on deep imaging of galaxies, addressing challenges, and discussing the optimization of an observing strategy coupled to a dedicated data reduction pipeline. With new territories in signal level being explored (~28.5 mag/arcsec² in direct detection and 32 mag/arcsec² on integrated profiles) I will then address a number of limiting issues such as optical reflection halos, stray light, airglow, and the ubiquitous cirrus emission.