How Many Subs

M91

NGC 4548 · Galaxy in Coma Berenices

Magnitude
10.2
Size
5.4′ × 4.3′
Effective surface brightness
22.2 mag/arcsec²
Best months
Mar, Apr, May
Declination
+15°
Filter
None (broadband target)

A faint barred spiral, the last Virgo cluster object Messier catalogued and the hardest of the group.

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 M91, default sub length, recommended filter, and how much of each frame it fills. Click a cell for the full Bortle 1 to 9 table.
TelescopeIn frameBortle 3 (rural)Bortle 5 (suburban)Bortle 7 (urban edge)
ZWO Seestar S5010 s subs13%1 h 29 min532 subs2 h 25 min872 subs6 h 34 min2,363 subs
ZWO Seestar S50 Pro10 s subs7%1 h 39 min592 subs2 h 40 min961 subs7 h 09 min2,573 subs
ZWO Seestar S3010 s subs8%1 h 40 min602 subs2 h 37 min943 subs6 h 45 min2,433 subs
ZWO Seestar S30 Pro10 s subs4%1 h 46 min635 subs2 h 50 min1,023 subs7 h 33 min2,718 subs
DwarfLab DWARF 315 s subs5%1 h 55 min462 subs3 h 23 min813 subs9 h 47 min2,348 subs
DwarfLab DWARF mini15 s subs8%1 h 27 min350 subs2 h 24 min578 subs6 h 33 min1,571 subs
DwarfLab Draco30 s subs7%1 h 11 min142 subs1 h 58 min237 subs5 h 25 min651 subs
DwarfLab DWARF II15 s subs5%6 h 35 min1,582 subs9 h 13 min2,214 subs21 h4,974 subs
Vaonis Vespera II10 s subs6%1 h 29 min532 subs2 h 25 min872 subs6 h 34 min2,363 subs
Vaonis Vespera Pro10 s subs6%3 h 38 min1,307 subs5 h 37 min2,024 subs14 h5,158 subs
Vaonis Vespera 310 s subs6%1 h 24 min503 subs2 h 18 min831 subs6 h 17 min2,262 subs
Vaonis Vespera Pro 210 s subs5%3 h 25 min1,233 subs5 h 20 min1,921 subs14 h4,931 subs
Unistellar eQuinox 24 s subs16%2 h 01 min1,817 subs2 h 37 min2,348 subs5 h 11 min4,671 subs
Unistellar Odyssey4 s subs16%11 h9,729 subs13 h11,662 subs22 h20,111 subs
Celestron Origin10 s subs11%21 min125 subs37 min221 subs1 h 47 min644 subs
Celestron Origin Mark II10 s subs12%26 min154 subs49 min293 subs2 h 30 min902 subs

Open in the calculator

Imaging the galaxy

Galaxies are broadband targets: their light is mostly starlight spread across the spectrum, so a dual-band filter throws most of it away and makes things slower, not faster. Shoot without a filter (or with a plain UV/IR cut) and rely on total time and a dark sky. The core of a galaxy is usually many times brighter than its outer disc, so a short stack shows the nucleus long before the arms; the figures here are for the arms and disc, not the core.

Field rotation in Alt-Az mode crops the edges of the stack over a long session, so on a big galaxy re-centre now and then or use EQ mode where your scope supports it.

Per-telescope pages