Go / no go for deep skyGo / no go for deep sky imaging
M21
open cluster in Sagittarius
M21 is a compact open cluster in Sagittarius, less than a degree from the Trifid Nebula (M20). The two share a field, with the cluster's blue-white stars sitting beside the nebula's red and blue glow.
Save your telescope or lens on the planner to lay your camera frame over this diagram.
// Key facts
- Catalog
- M21
- Type
- open cluster
- Constellation
- Sagittarius
- Size
- 13′ across
- Brightness
- Magnitude 6.5
- Distance
- about 4,250 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 M21
- A frame of about 1.5 degrees (roughly 700 mm on APS-C) shows M20 and M21 together.
- As a standalone target it is small, at 13 arcminutes, so it makes sense as part of the Trifid frame.
- Dec -22 is low from northern sites; wait for the meridian crossing.
Framing: what focal length?
M21 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 16% |
| 2,000 mm | about 32% |
| 3,000 mm | about 48% |
How high does it climb from your latitude?
The highest point M21 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) | 13° | Stays low |
| 45°N (Minneapolis, Milan) | 23° | Stays low |
| 35°N (Los Angeles, Tokyo) | 33° | Workable |
| 25°N (Miami, Cairo) | 43° | Workable |
| 35°S (Sydney, Santiago) | 78° | High in the sky |
Also worth imaging
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