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Dolphin Head Nebula (Sh2-308)
emission nebula in Canis Major
The Dolphin Head Nebula is a faint, round emission shell in Canis Major, blown by the strong wind of a hot Wolf-Rayet star. It is large and faint, and needs OIII to show properly. It is a fine target for a southern or low-latitude observer.
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// Key facts
- Catalog
- Sh2-308
- Also known as
- Sharpless 308, Sh 2-308
- Type
- emission nebula
- Constellation
- Canis Major
- Size
- 40′ across
- Brightness
- Magnitude 10
- Distance
- about 5,000 light-years
- Peak month (any location)
- Jan
- Typical prime months
- Oct to Mar
- Difficulty
- Advanced
- Filter
- Narrowband: Ha / OIII / SII, or a dual-band / UHC filter
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How to image Dolphin Head Nebula
- Use 400 to 800 mm; it is about 40 arcminutes across.
- OIII is the key filter; the nebula is very faint in Ha.
- It only gets about 30 degrees up from 35 degrees north, so shoot near the meridian in winter.
Framing: what focal length?
The focal length at which Sh2-308 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 680 mm |
| APS-C (23.5 mm) | about 1210 mm |
| Full frame (36 mm) | about 1860 mm |
How high does it climb from your latitude?
The highest point Sh2-308 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) | 11° | Stays low |
| 45°N (Minneapolis, Milan) | 21° | Stays low |
| 35°N (Los Angeles, Tokyo) | 31° | Workable |
| 25°N (Miami, Cairo) | 41° | Workable |
| 35°S (Sydney, Santiago) | 79° | High in the sky |
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