M18
Open Cluster in Sagittarius
About
A sparse open cluster in Sagittarius, about 4,200 light-years away and roughly 33 million years old, sometimes called the Black Swan Cluster. It sits about 1° south of the Omega Nebula (M17).
Imaging tips
- Only about 9′ across, so it needs a long focal length: roughly 2,700–4,500 mm on APS-C, too small to frame on full frame.
- It is a cluster, so broadband colour is the choice; one imager's 6 frames of 6 minutes through a light-pollution filter on a 700 mm refractor were enough to record it.
- Against the dense Milky Way star fields of Sagittarius the cluster does not stand out sharply, so framing and star-field context shape how it reads in the picture.
- M17 is about 56′ away, so a wider frame at moderate focal length can take both. Best around June and July, below 23° altitude from 50°N.
Facts
- Size
- 9′
- Magnitude
- 6.9
- Coordinates
- RA 18h 20.0m, Dec −17° 5′
- Framing, APS-C
- 2690–4490 mm
- Framing, full frame
- longer than 4000 mm
- Best months
- Jun, Jul
- Peak altitude
- 23° from 50°N, 38° from 35°N