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Contains:  NGC 2395, NGC2395, PK205+14.1, Sh2-274
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SH2-274 - Medusa Nebula (HOO - LRGB stars), 


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

Technical card

Dates:Feb. 3, 2020Feb. 14, 2020Feb. 15, 2020Feb. 19, 2020Feb. 20, 2020Feb. 22, 2020

ZWO B 1.25" optimized for ASI1600: 45x90" (gain: 139.00) -20C bin 1x1
ZWO G 1.25" optimized for ASI1600: 42x90" (gain: 139.00) -20C bin 1x1
ZWO Ha 7nm 1.25": 107x180" (gain: 139.00) -20C bin 1x1
ZWO L 1.25" optimized for ASI1600: 75x90" (gain: 139.00) -20C bin 1x1
ZWO O-III 7nm 1.25": 80x180" (gain: 139.00) -20C bin 1x1
ZWO R 1.25" optimized for ASI1600: 37x90" (gain: 139.00) -20C bin 1x1

Integration: 14.3 hours

Darks: ~12

Flats: ~15

Bias: ~20

Avg. Moon age: 21.97 days

Avg. Moon phase: 35.20%

Bortle Dark-Sky Scale: 5.00

Temperature: -3.00 job: 3289534

RA center: 7h 28' 58"

DEC center: +13° 18' 16"

Pixel scale: 1.205 arcsec/pixel

Orientation: 168.702 degrees

Field radius: 0.852 degrees

Resolution: 4097x3024

Locations: Happy Frog Observatory, Monroe, Connecticut, United States

Data source: Backyard


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.
Dates: 2-3-20, 2-14-20, 2-15-20, 2-19-20, 2-20-20, 2-21-20, 2-22-20



Kurt Zeppetello
License: None (All rights reserved)


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


            Kurt Zeppetello
  • Final
    SH2-274 - Medusa Nebula (HOO - LRGB stars), 


            Kurt Zeppetello


Description: I added a little color, I originally thought I did this.

Sky plot

Sky plot


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


            Kurt Zeppetello

In these public groups

Imaged with APT
Northeast Imagers