When to photograph M 7 (Ptolemy's Cluster)
M 7, the Ptolemy's Cluster, is an open cluster in Scorpius, 22′ across at magnitude 3.3. Also listed as NGC 6475.
When to photograph it
From 45° N it never climbs above 30°. It peaks at 10° when it crosses the meridian, low enough that haze and light pollution soften it. It is possible with a clear southern horizon, but hard.
From 35° S the best months are May to July, with up to 10.0 hours of astronomical darkness above 30° in June. It reaches 90° at its highest.
Framing
Its long axis is 22′. It fills 35 to 80% of the short side of an APS-C sensor at about 850 to 1930 mm, and of a full-frame sensor at about 1300 to 2970 mm.
Filters and the moon
Clusters are broadband targets, but bright ones tolerate some moonlight and light pollution. Short subs keep the brightest stars from clipping. The dark sky calendar shows which nights this month are moon-free where you are.
Plan it for tonight
Dates and times depend on your exact location. The imaging planner shows M 7's altitude through tonight, its transit time and its distance from the moon. For sub length, use the exposure calculator.
Nearby targets
- M 6 Butterfly Cluster 3.8° away
- NGC 6357 Lobster Nebula 6.0° away
- NGC 6334 Cat's Paw Nebula 6.8° away
- M 69 8.2° away
- NGC 6302 Bug Nebula 8.4° away
- NGC 6541 9.3° away
Gear that suits M 7
M 7 is 22′ along its long axis. These frame it on an APS-C sensor with room to crop.
- Sky-Watcher Esprit 120ED 840 mm f/7: fills 35% of an APS-C frame's short side View at High Point Scientific
- Sky-Watcher Quattro 200P 800 mm f/4: fills 33% of an APS-C frame's short side View at High Point Scientific
- Celestron C8 OTA 2032 mm f/10: fills 84% of an APS-C frame's short side View at High Point Scientific
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