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Double Cluster (NGC 869)
open cluster in Perseus
The Double Cluster is a pair of rich, young open clusters, NGC 869 and NGC 884, sitting side by side in Perseus. Both are visible to the naked eye as a hazy patch, and the stars include bright blue giants and a few red supergiants, giving the field good color.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- NGC 869
- Type
- open cluster
- Constellation
- Perseus
- Size
- 1.0° across
- Brightness
- Magnitude 4.3
- Distance
- about 7,500 light-years
- Peak month (any location)
- Oct
- Typical prime months
- Aug to Jan
- Difficulty
- Beginner
- Filter
- No filter (broadband)
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How to image Double Cluster
- Each cluster is about half a degree across, so a frame of 1.5 to 2 degrees (around 500 mm on APS-C) fits both.
- Short exposures are enough, and star color is the main reward, so take care to avoid saturating the stars.
- High in the northern sky for much of the year, and a good target when the moon is up.
Framing: what focal length?
The focal length at which NGC 869 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 450 mm |
| APS-C (23.5 mm) | about 810 mm |
| Full frame (36 mm) | about 1240 mm |
How high does it climb from your latitude?
The highest point NGC 869 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) | 88° | High in the sky |
| 45°N (Minneapolis, Milan) | 78° | High in the sky |
| 35°N (Los Angeles, Tokyo) | 68° | High in the sky |
| 25°N (Miami, Cairo) | 58° | Workable |
| 35°S (Sydney, Santiago) | below horizon | Never rises |
Also worth imaging
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