SH2-274 - Medusa Nebula (HOO - LRGB stars)

This neat looking object is known as the Medusa Nebula as it bears a resemblance to the snake-like hair of Medusa in Greek myths.  It is also known as SH2-274 of the Sharpless catalog and Abell 21.  It is located approximately 1500 ly away in the constellation of Gemini and may be my first Abell object!  Originally this was thought to have been a supernova remnant, however, research done by Soviet astronomers concluded it was a planetary nebula.

A planetary nebula represents the final phase of a sun-like star, first transforming to a red giant, then to a hot white dwarf while releasing its outer layers. The Medusa is estimated to be 4 light-years across, roughly the same distance to the nearest star to our own sun (Proxima Centauri).  The reddish colors are due to the large amounts of hydrogen gas while blue-green is from ionized oxygen.  There is a least one small galaxy visible in the upper right background but there may be more.

I was going to do a straight LRGB version as I thought would be too small to see any detail but I changed my mind - moon and weather issues.  Doing the Ha-OIII brought out the faint dusty gas area in the lower left.  I really like the colorful star field so merged it with the HOO nebula using a star mask in PI.  After several bending iterations I determined that red - Ha (100%), green Ha (30%) + OIII(70%), and blue (100%) gave the optimal detail and color variation.  The image was cropped and probably could be cropped further, in fact many people would, but I like the star field it sits in so I wound up with this compromise.

SH2-274 (Abell 21) - Medusa Nebula
Home Monroe, CT
Date: 2-3-20, 2-14-20, 2-15-20, 2-19-20, 2-20-20, 2-21-20, 2-22-20
Camera: ZWO ASI1600MM-Pro
Telescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Barlow: None
Focal Length: 805mm
f/7
Focal Reducer: Astro-Tech 0.8x Focal Reducer/Field Flattener for Refractor Telescopes
Mount: Orion Sirius
Filter Wheel: ZWO EFW 8 x 1.25"
Filter: ZWO Ha, OIII, L, R, G, B
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: Ha 107 x 180, OIII 80 x 180, L 75 x 90s, R 37 x 90s, G 42 x 90s, B 45 x 90s
Gain: 139
Offset 21
Temp: -4 C
Processing: APT, PixInsight, Photoshop, Lightroom.
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