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Medusa Nebula (Sh2-274)
planetary nebula in Gemini
The Medusa Nebula (Abell 21) is a large, extremely faint planetary nebula in Gemini whose twisted filaments look like snakes. It is wide, with a very low surface brightness, so it is one of the toughest in the catalog. Narrowband data and dark skies are the usual route.
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// Key facts
- Catalog
- Sh2-274
- Also known as
- Sharpless 274, Abell 21, Sh 2-274
- Type
- planetary nebula
- Constellation
- Gemini
- Size
- 10′ across
- Brightness
- Magnitude 11
- Distance
- roughly 1,500 to 2,000 light-years
- Peak month (any location)
- Jan
- Typical prime months
- Oct to Mar
- Difficulty
- Advanced
- Filter
- OIII or dual-band / UHC filter
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How to image Medusa Nebula
- It is about 10 arcminutes across, so 1,000 to 1,500 mm suits it.
- Use OIII and Ha; the glow is very faint and filters make the difference.
- Spend many hours; this is a long-haul project.
Framing: what focal length?
Sh2-274 is small, so a "60% of the frame" focal length would be impractical. Here is how much of an APS-C frame it covers at common long focal lengths; expect to crop.
| Focal length | Share of the frame width |
|---|---|
| 1,000 mm | about 12% |
| 2,000 mm | about 25% |
| 3,000 mm | about 37% |
How high does it climb from your latitude?
The highest point Sh2-274 reaches in the sky. Above about 60° is excellent, 30° to 60° is workable, and under 30° you are looking through a lot of air.
| Latitude | Highest point | Rating |
|---|---|---|
| 55°N (Scotland, Alaska) | 48° | Workable |
| 45°N (Minneapolis, Milan) | 58° | Workable |
| 35°N (Los Angeles, Tokyo) | 68° | High in the sky |
| 25°N (Miami, Cairo) | 78° | High in the sky |
| 35°S (Sydney, Santiago) | 42° | Workable |
Also worth imaging
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