Large astronomical telescopes are necessarily reflecting: it's more cost effective to build large mirrors than large lenses. But that necessarily implies that

Large astronomical telescopes are necessarily reflecting: it's more cost effective to build large mirrors than large lenses. But that necessarily implies that there will be obscuration (secondary mirrors, etc.) in the light path, which complicates the focused image and makes it difficult to look for diffuse, low-surface brightness features. The other challenge in looking for these sorts of objects is scattered light. What van Dokkum & co realized was that commercial telephoto lenses are exquisitely designed to minimize scattered light, and as refractors have no central obscuration. So a small array of such lenses is actually more sensitive to faint, diffuse features than much larger telescopes. (Plus cheaper!)