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Trifid Nebula (M20)
emission nebula in Sagittarius
The Trifid Nebula is unusual because it holds three kinds of nebula in one place: the red glow of an emission nebula, blue reflection nebula to the north, and dark dust lanes that split the red part into three lobes. That mix of colors makes it a rewarding target in full color as well as narrowband.
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// Key facts
- Catalog
- M20
- Type
- emission nebula
- Constellation
- Sagittarius
- Size
- 28′ across
- Brightness
- Magnitude 6.3
- Distance
- about 4,000 to 5,000 light-years
- Peak month (any location)
- Jun
- Typical prime months
- Apr to Sep
- Difficulty
- Intermediate
- Filter
- Narrowband: Ha / OIII / SII, or a dual-band / UHC filter
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How to image Trifid Nebula
- Because the blue part is reflected starlight, use broadband color for it. A dual-band filter can lose the blue.
- The open cluster M21 is only a little way away and fits in a wider frame.
- Pair it with the Lagoon Nebula in one frame at about 300 to 400 mm.
Framing: what focal length?
The focal length at which M20 spans about 60% of the long side of the sensor, leaving room around it. Treat it as a starting point; the right framing is a matter of taste.
| Sensor | Focal length |
|---|---|
| 1-inch sensor (13.2 mm) | about 970 mm |
| APS-C (23.5 mm) | about 1730 mm |
| Full frame (36 mm) | about 2650 mm |
How high does it climb from your latitude?
The highest point M20 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) | 12° | Stays low |
| 45°N (Minneapolis, Milan) | 22° | Stays low |
| 35°N (Los Angeles, Tokyo) | 32° | Workable |
| 25°N (Miami, Cairo) | 42° | Workable |
| 35°S (Sydney, Santiago) | 78° | High in the sky |
Also worth imaging
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