Ultra Low Surface Brightness Imaging with the Dragonfly Telephoto Array
  We describe the Dragonfly Telephoto Array, a robotic imaging system optimized
for the detection of extended ultra low surface brightness structures. The
array consists of eight Canon 400mm f/2.8 telephoto lenses coupled to eight
science-grade commercial CCD cameras. The lenses are mounted on a common
framework and are co-aligned to image simultaneously the same position on the
sky. The system provides an imaging capability equivalent to a 0.4m aperture
f/1.0 refractor with a 2.6 deg X 1.9 deg field of view. The system has no
obstructions in the light path, optimized baffling, and internal optical
surfaces coated with a new generation of anti-reflection coatings based on
sub-wavelength nanostructures. As a result, the array's point spread function
has a factor of ~10 less scattered light at large radii than well-baffled
reflecting telescopes. The Dragonfly Telephoto Array is capable of imaging
extended structures to surface brightness levels below 30 mag/arcsec^2 in 10h
integrations (without binning or foreground star removal). This is considerably
deeper than the surface brightness limit of any existing wide-field telescope.
At present no systematic errors limiting the usefulness of much longer
integration times has been identified. With longer integrations (50-100h),
foreground star removal and modest binning the Dragonfly Telephoto Array is
capable of probing structures with surface brightnesses below 32 mag/arcsec^2.
Detection of structures at these surface brightness levels may hold the key to
solving the "missing substructure" and "missing satellite" problems of
conventional hierarchical galaxy formation models. The Dragonfly Telephoto
Array is therefore executing a fully-automated multi-year imaging survey of a
complete sample of nearby galaxies in order to undertake the first census of
ultra-faint substructures in the nearby Universe.
