Go / no go for deep skyGo / no go for deep sky imaging
Summer Rose Star (M55)
globular cluster in Sagittarius
M55 is a large, loose globular cluster in Sagittarius, with a sparse core that makes it easier to resolve into stars than most. It is also quite faint for its size, so it looks soft rather than sharply concentrated.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- M55
- Type
- globular cluster
- Constellation
- Sagittarius
- Size
- 19′ across
- Brightness
- Magnitude 6.3
- Distance
- about 17,600 light-years
- Peak month (any location)
- Jul
- Typical prime months
- May to Sep
- Difficulty
- Intermediate
- Filter
- No filter (broadband)
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How to image Summer Rose Star
- It is large (19 arcminutes), so 600 to 1,000 mm frames it well.
- Dec -31 puts it low from most of the northern hemisphere, and southern sites see it much better.
- Because it is loose, it clips less than most globulars and is forgiving for exposure.
Framing: what focal length?
M55 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 24% |
| 2,000 mm | about 47% |
| 3,000 mm | about 71% |
How high does it climb from your latitude?
The highest point M55 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) | 4° | Stays low |
| 45°N (Minneapolis, Milan) | 14° | Stays low |
| 35°N (Los Angeles, Tokyo) | 24° | Stays low |
| 25°N (Miami, Cairo) | 34° | Workable |
| 35°S (Sydney, Santiago) | 86° | High in the sky |
Also worth imaging
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