M108

NGC 3556 · Galaxy in Ursa Major

Magnitude: 10.0
Size: 8.7′ × 2.2′
Effective surface brightness: 21.8 mag/arcsec²
Best months: Feb, Mar, Apr
Declination: +56°
Filter: None (broadband target)

An edge-on spiral with a mottled, dusty disc, a degree from the Owl Nebula. Frame both together.

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 M108, 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	20%	35 min211 subs	55 min333 subs	2 h 25 min870 subs
ZWO Seestar S50 Pro10 s subs	11%	39 min234 subs	1 h 01 min367 subs	2 h 38 min947 subs
ZWO Seestar S3010 s subs	12%	39 min236 subs	1 h359 subs	2 h 29 min896 subs
ZWO Seestar S30 Pro10 s subs	6%	42 min250 subs	1 h 05 min390 subs	2 h 47 min1,001 subs
DwarfLab DWARF 315 s subs	9%	46 min186 subs	1 h 18 min312 subs	3 h 36 min865 subs
DwarfLab DWARF mini15 s subs	12%	35 min139 subs	55 min221 subs	2 h 25 min579 subs
DwarfLab Draco30 s subs	12%	28 min57 subs	45 min91 subs	2 h240 subs
DwarfLab DWARF II15 s subs	8%	2 h 31 min605 subs	3 h 28 min832 subs	7 h 37 min1,827 subs
Vaonis Vespera II10 s subs	10%	35 min211 subs	55 min333 subs	2 h 25 min870 subs
Vaonis Vespera Pro10 s subs	9%	1 h 25 min511 subs	2 h 08 min769 subs	5 h 16 min1,898 subs
Vaonis Vespera 310 s subs	10%	33 min200 subs	53 min318 subs	2 h 19 min833 subs
Vaonis Vespera Pro 210 s subs	9%	1 h 20 min482 subs	2 h 02 min730 subs	5 h 02 min1,814 subs
Unistellar eQuinox 24 s subs	25%	46 min684 subs	58 min875 subs	1 h 54 min1,712 subs
Unistellar Odyssey4 s subs	26%	4 h 01 min3,611 subs	4 h 47 min4,307 subs	8 h 10 min7,351 subs
Celestron Origin10 s subs	17%	8 min51 subs	14 min85 subs	40 min238 subs
Celestron Origin Mark II10 s subs	19%	10 min63 subs	19 min114 subs	55 min333 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.

M108

How long with each smart telescope

Imaging the galaxy

Per-telescope pages