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Ptolemy Cluster (M7)
open cluster in Scorpius
Ptolemy's Cluster is a large, bright open cluster in Scorpius, known since antiquity and visible to the naked eye as a hazy patch. It spans more than a degree, so it needs a wide frame, and it sits in a dense Milky Way field.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- M7
- Type
- open cluster
- Constellation
- Scorpius
- Size
- 1.3° across
- Brightness
- Magnitude 3.3
- Distance
- about 1,000 light-years
- Peak month (any location)
- Jun
- Typical prime months
- Apr to Sep
- Difficulty
- Beginner
- Filter
- No filter (broadband)
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How to image Ptolemy Cluster
- At 80 arcminutes it needs a focal length of 200 to 400 mm on APS-C.
- Pair it with M6 in a very wide frame, or with the Lagoon Nebula (M8) region in a mosaic.
- Low from northern latitudes at dec -35, so wait for the meridian crossing.
Framing: what focal length?
The focal length at which M7 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 340 mm |
| APS-C (23.5 mm) | about 610 mm |
| Full frame (36 mm) | about 930 mm |
How high does it climb from your latitude?
The highest point M7 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) | below horizon | Never rises |
| 45°N (Minneapolis, Milan) | 10° | Stays low |
| 35°N (Los Angeles, Tokyo) | 20° | Stays low |
| 25°N (Miami, Cairo) | 30° | Workable |
| 35°S (Sydney, Santiago) | 90° | High in the sky |
Also worth imaging
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