Object: Constellation: Coordinates: Size: Brightness:
Abell 21
(=PK 205+14.1)
Gem 07h29m / +1315' 12' 16mag (?)

The planetary nebula Abell 21 ("Medusa Nebula", Sharpless 2-274, PK 205+14.1) in the constellation Gemini on December 20, 2015 and February 5 / 6, 2016. Abell 21 is 1500 light-years from Earth, four light-years across and was discovered by American astronomer George O. Abell in 1955. The filaments of glowing gas suggests the serpent hair of Medusa found in ancient Greek mythology (source: Wikipedia).

Twenty-seven exposures of approximately 5 minutes at ISO 800 with no dark frame subtraction were stacked with Deep Sky Stacker (resulting in a 2 h 24 min exposure) and further processed in Photoshop.
Equipment: Canon EOS 450D Baader modified camera, TeleVue Paracorr Type II coma corrector, 16" f/4.5 "Ninja" dobsonian telescope riding on a dual-axis Tom Osypowski equatorial platform, Lacerta MGEN autoguider, Lacerta off axis system (field of view comparison: image of the moon with the same equipment).

Search chart for Abell 21. Map © 2016 "The Mag-7 Star Atlas Project", www.siaris.net. Map is modified. The map can be downloaded here .