M21
NGC 6531 · Open cluster in Sagittarius
- Magnitude
- 5.9
- Size
- 13′
- Effective surface brightness
- 20.0 mag/arcsec²
- Best months
- Jun, Jul, Aug
- Declination
- -23°
- Filter
- None (broadband target)
A young open cluster beside the Trifid Nebula. Comes for free in a Trifid frame.
Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
How long with each smart telescope
| Telescope | In frame | Bortle 3 (rural) | Bortle 5 (suburban) | Bortle 7 (urban edge) |
|---|---|---|---|---|
| ZWO Seestar S5010 s subs | 30% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| ZWO Seestar S50 Pro10 s subs | 16% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| ZWO Seestar S3010 s subs | 18% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| ZWO Seestar S30 Pro10 s subs | 10% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| DwarfLab DWARF 315 s subs | 13% | 8 min30 subs | 8 min30 subs | 8 min30 subs |
| DwarfLab DWARF mini15 s subs | 18% | 8 min30 subs | 8 min30 subs | 8 min30 subs |
| DwarfLab Draco30 s subs | 17% | 15 min30 subs | 15 min30 subs | 15 min30 subs |
| DwarfLab DWARF II15 s subs | 12% | 8 min30 subs | 8 min31 subs | 13 min53 subs |
| Vaonis Vespera II10 s subs | 15% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| Vaonis Vespera Pro10 s subs | 13% | 5 min30 subs | 5 min31 subs | 9 min57 subs |
| Vaonis Vespera 310 s subs | 15% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| Vaonis Vespera Pro 210 s subs | 13% | 5 min30 subs | 5 min30 subs | 9 min54 subs |
| Unistellar eQuinox 24 s subs | 38% | 2 min30 subs | 2 min30 subs | 3 min49 subs |
| Unistellar Odyssey4 s subs | 39% | 8 min117 subs | 9 min133 subs | 13 min203 subs |
| Celestron Origin10 s subs | 25% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| Celestron Origin Mark II10 s subs | 29% | 5 min30 subs | 5 min30 subs | 5 min30 subs |
Imaging the open cluster
Open clusters are the easiest targets in the sky. The stars are bright, so the aim is colour and tight stars rather than depth: short subs, no filter, and stop before the bright members saturate. The nominal time here gets you clean star colour; more integration only adds background stars.
Per-telescope pages
- How long to image the M21 with the ZWO Seestar S50
- How long to image the M21 with the ZWO Seestar S50 Pro
- How long to image the M21 with the ZWO Seestar S30
- How long to image the M21 with the ZWO Seestar S30 Pro
- How long to image the M21 with the DwarfLab DWARF 3
- How long to image the M21 with the DwarfLab DWARF mini
- How long to image the M21 with the DwarfLab Draco
- How long to image the M21 with the DwarfLab DWARF II
- How long to image the M21 with the Vaonis Vespera II
- How long to image the M21 with the Vaonis Vespera Pro
- How long to image the M21 with the Vaonis Vespera 3
- How long to image the M21 with the Vaonis Vespera Pro 2
- How long to image the M21 with the Unistellar eQuinox 2
- How long to image the M21 with the Unistellar Odyssey
- How long to image the M21 with the Celestron Origin
- How long to image the M21 with the Celestron Origin Mark II