Corradi, R. L. M.; et al, Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties, Monthly Notices of the Royal Astronomical Society, April 2003, 340 (2): 417–446, Bibcode:2003MNRAS.340..417C, doi:10.1046/j.1365-8711.2003.06294.x.
Corradi, R. L. M.; et al, Ionized haloes in planetary nebulae: new discoveries, literature compilation and basic statistical properties, Monthly Notices of the Royal Astronomical Society, April 2003, 340 (2): 417–446, Bibcode:2003MNRAS.340..417C, doi:10.1046/j.1365-8711.2003.06294.x.
illinois.edu
stars.astro.illinois.edu
Kaler, James B., NGC 2022 in Orion, STARS, [2020-04-12], (原始内容存档于2019-03-01).
spacetelescope.org
The Inky Abyss. www.spacetelescope.org. [12 August 2019]. (原始内容存档于2019-09-30) (英语).