G.O. Abell. Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey. „Publications of the Astronomical Society of the Pacific”. 67 (397), s. 258-261, 1955. DOI: 10.1086/126815. Bibcode: 1955PASP...67..258A. (ang.).
G.O. Abell. Globular Clusters and Planetary Nebulae Discovered on the National Geographic Society-Palomar Observatory Sky Survey. „Publications of the Astronomical Society of the Pacific”. 67 (397), s. 258-261, 1955. DOI: 10.1086/126815. Bibcode: 1955PASP...67..258A. (ang.).