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Cave Nebula (Sh2-155)
emission nebula in Cepheus
The Cave Nebula is a faint emission nebula in Cepheus, shaped like the dark mouth of a cave, with a bright rim. It is one of the more elusive Caldwell objects, and it needs narrowband imaging. A dark sky helps a lot.
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// Key facts
- Catalog
- Sh2-155
- Also known as
- C 9, Caldwell 9, Sharpless 155, Sh 2-155
- Type
- emission nebula
- Constellation
- Cepheus
- Size
- 50′ across
- Brightness
- Magnitude 7.7
- Distance
- about 2,400 light-years
- Peak month (any location)
- Sep
- Typical prime months
- Jun to Nov
- Difficulty
- Advanced
- Filter
- Narrowband: Ha / OIII / SII, or a dual-band / UHC filter
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How to image Cave Nebula
- It is about 50 arcminutes across; 400 to 800 mm frames it well.
- Hydrogen-alpha or a dual-band filter shows it; broadband alone usually loses it.
- Cepheus is high in the north from northern sites most of the year.
Framing: what focal length?
The focal length at which Sh2-155 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 540 mm |
| APS-C (23.5 mm) | about 970 mm |
| Full frame (36 mm) | about 1490 mm |
How high does it climb from your latitude?
The highest point Sh2-155 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) | 82° | High in the sky |
| 45°N (Minneapolis, Milan) | 72° | High in the sky |
| 35°N (Los Angeles, Tokyo) | 62° | High in the sky |
| 25°N (Miami, Cairo) | 52° | Workable |
| 35°S (Sydney, Santiago) | below horizon | Never rises |
Also worth imaging
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