When to photograph M 21
M 21 is an open cluster in Sagittarius, 6′ across at magnitude 5.9. Also listed as NGC 6531.
When to photograph it
From 45° N it never climbs above 30°. It peaks at 23° 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 9.0 hours of astronomical darkness above 30° in June. It reaches 77° at its highest.
Framing
Its long axis is 6′. It fills 35 to 80% of the short side of an APS-C sensor at about 3130 to 7150 mm, and of a full-frame sensor at about 4810 to 11000 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 21's altitude through tonight, its transit time and its distance from the moon. For sub length, use the exposure calculator.
Nearby targets
- M 20 Trifid Nebula 0.6° away
- M 8 Lagoon Nebula 1.9° away
- M 23 3.9° away
- M 24 Sagittarius Star Cloud 5.0° away
- M 28 5.2° away
- M 18 6.5° away
Gear that suits M 21
M 21 is only 6.0′ across, so it stays small in any amateur telescope. The longest focal length gives it the most pixels.
- Celestron C8 OTA 2032 mm f/10: fills 23% of an APS-C frame's short side View at High Point Scientific
Retailer links are affiliate links. If you buy through one, the site may earn a commission at no extra cost to you. Details