Abell 21 (PN A66 21, PK 205+14.1, PN G205.1+14.2), also known as the Medusa Nebula is a highly evolved
planetary nebula in Gemini. It has also been cataloged as an HII region by Sharpless as Sh2-274.
The bluish star in the center is the associated white dwarf (WD0017+283). Taken Dec 23, 2009, Jan
9, Feb 16, 17, Mar 13 and 14, 2010. Imaged with CCD Labís Q285M Camera and a C8N at f5 on a Losmandy
G11/Gemini mount. The H-alpha and OIII subframes were 8 minutes, while the RGB frames were 2
minutes. The final total exposures were (Ha+OIII) : (R+Ha) : (G+OIII) : (B+OIII) (min) = 240+240 :
54+240 : 60+240 : 60+240. Color balance was achieved via numerically scaled channels and with the
help of a G type field star. Captured with Nebulosity, calibrated and sigma combined in Iris with
other processing using PixInsight LE and Photoshop CS. North is up.