Most consider Roberts' "magnum opus" to be a photograph showing the structure of M31, the Great Nebula in Andromeda (now known as the Andromeda Galaxy). He made the photo on 29 December 1888, using his 20-inch aperture reflecting telescope made by Howard Grubb of Dublin. The long exposure photograph revealed that the nebula had a spiral structure, which was quite unexpected at the time. Photographs such as this changed astronomy by revealing the true form of nebulae and clusters, and eventually helped to develop the theories about galaxies. He published his celestial portfolio in a large format book that is the first popular account of celestial photography of the deep sky.