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Our knowledge of the properties of disks of galaxies has been driven by deep imaging for the past decades. Until the mid-1980's, such imaging was done with photographic plates, prepared using specialist chemical techniques and then exposed for long times on large telescopes, often by observers spending long and uncomfortable hours in the prime focus cage of the telescope. The early history of this, and the subsequent physical parameters derived for disk galaxies, have been summarized by, e.g., van der Kruit and Freeman (2011). Main findings relating to the structure of disks include the description of the surface brightness distribution of disks as exponential by Freeman (1970), and the realization that the vertical profiles of disks seen edge-on can be described by an isothermal sheet (van der Kruit and Searle 1981). Although in the early days they were limited by a small field-of-view (FOV) and flat fielding issues, imaging with charge-coupled devices (CCDs) quickly took over from photographic plates. Large imaging surveys now provide most data, as reviewed below.

A powerful alternative to deep imaging of integrated light from galaxies is imaging and characterizing individual stars in the outskirts of a galaxy, either by using a camera with a large FOV to observe Local Group galaxies like M31, or by using the Hubble Space Telescope (HST) to observe dwarf galaxies in the Local Group or to resolve the stars in galaxies outside the Local Group but at distances smaller than 16 Mpc (Zackrisson et al 2012). Exploration in this sense of M31 started with the Isaac Newton Telescope Wide Field Camera survey (Ibata et al 2001) and has been recently reviewed by Ferguson and Mackey (2016). Key references for other nearby galaxies include Dalcanton et al (2009), Radburn-Smith et al (2011), Gallart et al (2015) and Monachesi et al (2016). In the current Chapter, we will concentrate on imaging of integrated light, and refer the interested reader to the Chapter by Crnojevic (this volume) for a review of results based on imaging individual stars.

Over the past decade, advances in detector technology, observing strategies, and data reduction procedures have led to the emergence of new lines of research based on what we call here ultra-deep imaging. This can be obtained in large imaging surveys, or obtained for small samples of galaxies, or for individual ones, using very long exposure times on small telescopes, or with large professional telescopes. In this Chapter, we will give examples of the first and third category, whereas impressive examples of results from the second category can be found in the Chapter by Abraham et al (this volume).

In this Chapter, we will first discuss the various challenges that need to be overcome before ultra-deep imaging can be used to distill scientific advances, in particular those due to light scattered in the atmosphere and telescope, flat fielding, and sky subtraction. We will then briefly review the main approaches to obtain deep imaging, namely from imaging surveys, using small telescopes, and using largeprofessional telescopes. In Sect. 4, which forms the heart of this review, we consider the progress in our understanding of the outskirts of galaxies that has been achieved thanks to ultra-deep imaging, paying particular attention to the properties of galaxy disks (including thick disks and disk truncations) and stellar haloes, but also touching on properties of tidal streams and satellites. When concluding, we will describe future developments, challenges and expected advances.

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