M7 — Ptolemy Cluster
Open Cluster in Scorpius
About
Ptolemy's Cluster, a bright open cluster in Scorpius about 980 light-years away, recorded by Ptolemy in 130 AD. It has over a thousand likely members spread across several degrees and is the southernmost Messier object.
Imaging tips
- About 80′ across, so it fills a typical frame at 300–500 mm on APS-C (460–770 mm on full frame).
- One imager spent 10 hours on 600-second subs with a fast astrograph to bring out the faint gas and dust behind the cluster, and suggests 300–600 s subs for that.
- For the cluster alone, the same write-up says 30-second subs over 1–2 hours are enough, and no filter is needed from a dark site.
- The field is very dusty, and the faint background is hard to reveal and balance in processing.
- Very low: it stays below 6° altitude from 50°N and reaches 20° from 35°N. Best in June and July.
Facts
- Size
- 80′
- Magnitude
- 3.3
- Coordinates
- RA 17h 53.9m, Dec −34° 49′
- Framing, APS-C
- 300–500 mm
- Framing, full frame
- 460–770 mm
- Best months
- Jun, Jul
- Peak altitude
- 5° from 50°N, 20° from 35°N