Abell 21, also known as Sharpless 2-274 and the "Medusa Nebula" is a planetary nebula in the constellation of Gemini. It was discovered in 1955 by George O. Abell, who classified it as an old planetary nebula. Until the early 1970s, it was thought to be a supernova remnant. With the computation of expansion velocities and the thermal character of the radio emission, in 1971 Soviet astronomers concluded that it was most likely a planetary nebula.The images comprise 5 x Ha, OIII and SII (300 seconds each), 0.5m f/2.9 ASA Astrograph with FLI ML3200 camera at an altitude of 70 degrees on 16 November 2017. The fields of view are 34’ x 23’ and 18' x 12', respectively. The Canada France Hawaii Telescope colour scheme has been adopted, Ha mapped to R, OIII mapped to G and SII to B.
Hills Observatory: 1 January 2013 to 16 November 2018