M4

NGC 6121 · Globular cluster in Scorpius

Magnitude: 5.9
Size: 36′
Effective surface brightness: 22.1 mag/arcsec²
Best months: May, Jun, Jul
Declination: -27°
Filter: None (broadband target)

The loose globular next to Antares. Low in the sky from the UK, so image it near transit.

Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.

How long with each smart telescope

Total integration for a solid image of the M4, default sub length, recommended filter, and how much of each frame it fills. Click a cell for the full Bortle 1 to 9 table.
Telescope	In frame	Bortle 3 (rural)	Bortle 5 (suburban)	Bortle 7 (urban edge)
ZWO Seestar S5010 s subs	84%	29 min176 subs	47 min284 subs	2 h 06 min756 subs
ZWO Seestar S50 Pro10 s subs	43%	32 min195 subs	52 min312 subs	2 h 17 min823 subs
ZWO Seestar S3010 s subs	50%	33 min198 subs	51 min306 subs	2 h 10 min779 subs
ZWO Seestar S30 Pro10 s subs	27%	35 min209 subs	55 min332 subs	2 h 25 min870 subs
DwarfLab DWARF 315 s subs	36%	38 min154 subs	1 h 06 min265 subs	3 h 08 min752 subs
DwarfLab DWARF mini15 s subs	50%	29 min116 subs	47 min188 subs	2 h 06 min503 subs
DwarfLab Draco30 s subs	48%	23 min47 subs	38 min77 subs	1 h 44 min209 subs
DwarfLab DWARF II15 s subs	33%	2 h 08 min514 subs	2 h 59 min715 subs	6 h 37 min1,590 subs
Vaonis Vespera II10 s subs	42%	29 min176 subs	47 min284 subs	2 h 06 min756 subs
Vaonis Vespera Pro10 s subs	37%	1 h 11 min429 subs	1 h 49 min656 subs	4 h 35 min1,650 subs
Vaonis Vespera 310 s subs	41%	28 min166 subs	45 min270 subs	2 h 01 min724 subs
Vaonis Vespera Pro 210 s subs	36%	1 h 07 min405 subs	1 h 44 min623 subs	4 h 23 min1,578 subs
Unistellar eQuinox 24 s subs	1.1× frame	39 min587 subs	50 min755 subs	1 h 39 min1,492 subs
Unistellar Odyssey4 s subs	1.1× frame	3 h 28 min3,124 subs	4 h 09 min3,737 subs	7 h 08 min6,417 subs
Celestron Origin10 s subs	70%	7 min42 subs	12 min72 subs	34 min207 subs
Celestron Origin Mark II10 s subs	80%	9 min52 subs	16 min96 subs	48 min289 subs

Open in the calculator

Imaging the globular cluster

Globular clusters are point sources packed tight, so they come quickly: the core resolves into stars within a handful of minutes on any smart scope. More time deepens the halo and pulls in the faint outer members. Use short subs so the core does not saturate into a white blob, and no filter.

M4

How long with each smart telescope

Imaging the globular cluster

Per-telescope pages