How long to image the M108 with the ZWO Seestar S50
From a Bortle 6 suburban sky, the ZWO Seestar S50 needs about 1 h 26 min (518 subs of 10 s) for a solid image of the M108. A quick look takes 22 min; a showpiece 5 h 45 min. Under a Bortle 3 rural sky the solid figure drops to 35 min. At Bortle 6 the limit is skyglow, so a darker site or a filter helps more than longer subs. In this scope's field it fills 20% of the frame’s short side.
| Sky | Quick lookTarget obvious, still noisy | SolidClean enough to share | ShowpieceSmooth faint detail, print-worthy |
|---|---|---|---|
| Bortle 1Excellent dark site | 8 min46 subs | 30 min181 subs | 2 h721 subs |
| Bortle 2Typical dark site | 8 min48 subs | 32 min191 subs | 2 h 07 min762 subs |
| Bortle 3Rural | 9 min53 subs | 35 min211 subs | 2 h 20 min841 subs |
| Bortle 4Rural / suburban transition | 11 min65 subs | 43 min258 subs | 2 h 52 min1,031 subs |
| Bortle 5Suburban | 14 min84 subs | 55 min333 subs | 3 h 42 min1,332 subs |
| Bortle 6Bright suburban | 22 min130 subs | 1 h 26 min518 subs | 5 h 45 min2,071 subs |
| Bortle 7Suburban / urban transition | 36 min218 subs | 2 h 25 min870 subs | 9 h 40 min3,479 subs |
| Bortle 8City | 59 min355 subs | 3 h 56 min1,417 subs | 16 h5,666 subs |
| Bortle 9Inner city | 1 h 48 min646 subs | 7 h 10 min2,583 subs | 29 h10,330 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 1 h 52 min of run time. The Seestar S50’s battery (rated about 6 h) should cover it; a power bank still keeps the phone running the app alive. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB-C power bank 20000mAh 5V 3AThe S50 draws 5 V 3 A over USB-C and runs while charging; no PD needed.
- Seestar S50 dew shieldThe built-in heater copes with most nights; a shield helps in heavy dew and blocks stray light.
- red LED head torchRed light keeps your night vision; white light costs you twenty minutes of dark adaptation every time.
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Everything that fits the Seestar S50 →
How it frames
The M108 is 8.7′ × 2.2′ across. The Seestar S50 frames 1.28° × 0.72° at 2.39″ per pixel, so it fills 20% of the frame’s short side.
Longer subs
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode (app 2.5 and later). Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 35 min211 subs | 55 min333 subs | 2 h 25 min870 subs |
| 20 s | 30 min91 subs | 51 min153 subs | 2 h 20 min421 subs |
| 30 s | 29 min58 subs | 49 min99 subs | 2 h 19 min278 subs |
| 60 s | 30 min30 subs | 47 min48 subs | 2 h 17 min137 subs |
Leave the dual-band filter out for the M108. It is a galaxy, and it shines right across the spectrum. A dual-band filter keeps two emission lines and throws the rest away, so most of the light you came for is being stopped along with the sky. From a Bortle 6 sky it would be roughly 12 times slower.
About the M108
An edge-on spiral with a mottled, dusty disc, a degree from the Owl Nebula. Frame both together.
NGC 3556 · Galaxy in Ursa Major · magnitude 10.0 · 8.7′ × 2.2′ · best in Feb, Mar, Apr. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
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.
About the ZWO Seestar S50
The scope that made smart astrophotography mainstream. 50 mm triplet, 2 MP sensor, dual-band filter on a wheel. Alt-Az stacking at 10 s, EQ mode unlocks 60 s. Out of production since 2026 and replaced by the S50 Pro, but there are a great many in use.
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode (app 2.5 and later). Built-in dual-band light-pollution filter, switchable in the app.
Longer subs matter most under dark skies, where read noise rather than skyglow sets the floor. From a city the difference between 10 s and 60 s subs is small; from a rural site it can halve the total time, especially with a dual-band filter in place.
Stack across nights. Every smart scope app can continue a stack or you can combine the saved frames on a computer; the figures here are totals, not single-session limits.
Have an image of the M108 from a Seestar S50? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- M108 with the ZWO Seestar S50 Pro: 1 h 34 min from Bortle 6
- M108 with the ZWO Seestar S30: 1 h 30 min from Bortle 6
- M108 with the ZWO Seestar S30 Pro: 1 h 40 min from Bortle 6
- M108 with the DwarfLab DWARF 3: 2 h 06 min from Bortle 6
- M108 with the DwarfLab DWARF mini: 1 h 26 min from Bortle 6
- M108 with the DwarfLab Draco: 1 h 11 min from Bortle 6
- M108 with the DwarfLab DWARF II: 4 h 54 min from Bortle 6
- M108 with the Vaonis Vespera II: 1 h 26 min from Bortle 6
- M108 with the Vaonis Vespera Pro: 3 h 13 min from Bortle 6
- M108 with the Vaonis Vespera 3: 1 h 22 min from Bortle 6
- M108 with the Vaonis Vespera Pro 2: 3 h 04 min from Bortle 6
- M108 with the Unistellar eQuinox 2: 1 h 18 min from Bortle 6
- M108 with the Unistellar Odyssey: 5 h 57 min from Bortle 6
- M108 with the Celestron Origin: 23 min from Bortle 6
- M108 with the Celestron Origin Mark II: 31 min from Bortle 6
All telescopes for the M108 · All targets for the Seestar S50