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.
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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).
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