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M18
open cluster in Sagittarius
M18 is a small, sparse open cluster in Sagittarius, just north of the Omega Nebula (M17), so it often turns up at the edge of Omega Nebula frames. It is modest in itself but sits in a spectacular field of star clouds.
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// Key facts
- Catalog
- M18
- Type
- open cluster
- Constellation
- Sagittarius
- Size
- 9′ across
- Brightness
- Magnitude 7.5
- Distance
- about 4,900 light-years
- Peak month (any location)
- Jun
- Typical prime months
- Apr to Sep
- Difficulty
- Intermediate
- Filter
- No filter (broadband)
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How to image M18
- It is only 9 arcminutes across, so it is mostly a frame companion to M17.
- A frame of about 1.5 to 2 degrees includes M17, M18 and the northern end of M24.
- Dec -17 makes it moderately low from northern sites.
Framing: what focal length?
M18 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 11% |
| 2,000 mm | about 22% |
| 3,000 mm | about 33% |
How high does it climb from your latitude?
The highest point M18 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) | 18° | Stays low |
| 45°N (Minneapolis, Milan) | 28° | Stays low |
| 35°N (Los Angeles, Tokyo) | 38° | Workable |
| 25°N (Miami, Cairo) | 48° | Workable |
| 35°S (Sydney, Santiago) | 72° | High in the sky |
Also worth imaging
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