Abell 21 is a relatively large, low surface brightness planetary nebula in southern Gemini, close to the border with Canis Minor. The magnitude is quoted as 11.3, but the light is spread out over and area 11.7 arc minutes in diameter. Some vague traces of it are visible in a 17.5" telescope.
Abell planetaries have a well-deserved reputation of being amongst some of the toughest challenges for the visual observer. Abell 21 is one of the easier members of this list, so makes a good starting object for anyone contemplating observing these objects.