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 400 mm f/2.8 L IS II USM telephoto lenses coupled to eight science-grade commercial CCD cameras. The lenses are mounted on a common framework and are coaligned to simultaneously image the same position on the sky. The system provides an imaging capability equivalent to a 0.4 m aperture f/1.0 refractor with a 2.6° × 1.9° field of view. The system is driven by custom software for instrument control and robotic operation. Data is collected with noncommon optical paths through each lens, and with careful tracking of sky variations in order to minimize systematic errors that limit the accuracy of background estimation and flat-fielding. The system has no obstructions in the light path, optimized baffling, and internal optical surfaces coated with a new generation of antireflection coatings based on subwavelength 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 μB = 30 mag arcsec-2 in ~10 h 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 have been identified. With longer integrations (50-100 h), foreground star removal, and modest binning, the Dragonfly Telephoto Array is capable of probing structures with surface brightnesses below μB = 32 mag arcsec-2. The 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 multiyear imaging survey of a complete sample of nearby galaxies in order to undertake the first census of ultrafaint substructures in the nearby universe.