Abell 21 is a large, low surface brightness planetary nebula in Gemini. It's extremely subtle in the 18-inch Obsession, first emerging during a sweep through the area as a delicate brightening against an onyx sky. Lumicon UHC and OIII filters enhance the view of this nebulous cloud, with the OIII producing the best results. The sketch at left captures Abell 21's appearance at 141X in the big dob. It's centered, a broad arcing nebulosity that looks something like a horse's hoof print. The semi-circular form covers an area roughly 12' across. The northeast and southwest portions are a tad brighter. The east-southeast edge appears a bit squared off, suggesting the subtlest wisp of material separating from the main section. Among the 30 or so field stars framing this object, the brightest shine at 10th magnitude with most chiming in at 12th or 13th magnitude. You'll find Abell 21 five degrees north of 2.9 magnitude Beta (3) Canis Majoris or about 4.2 degrees southeast of 3.6 magnitude Lambda (54) Geminorum.