Abell, George O. (1955), "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, Bibcode:1955PASP...67..258A, doi:10.1086/126815
Abell, George O. (1955), "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, Bibcode:1955PASP...67..258A, doi:10.1086/126815
Acker, A.; Marcout, J.; Ochsenbein, F.; Stenholm, B.; Tylenda, R.; Schohn, C. (1992). "The Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. Parts I, II". The Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. Parts I, II. Garching, Almanya: European Southern Observatory (ESO). Bibcode:1992secg.book.....A. ISBN3-923524-41-2.