How long to image the M108 with the ZWO Seestar S30
From a Bortle 6 suburban sky, the ZWO Seestar S30 needs about 1 h 30 min (543 subs of 10 s) for a solid image of the M108. A quick look takes 23 min; a showpiece 6 h 02 min. Under a Bortle 3 rural sky the solid figure drops to 39 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 12% 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 | 9 min52 subs | 34 min206 subs | 2 h 17 min822 subs |
| Bortle 2Typical dark site | 9 min54 subs | 36 min216 subs | 2 h 24 min864 subs |
| Bortle 3Rural | 10 min59 subs | 39 min236 subs | 2 h 37 min942 subs |
| Bortle 4Rural / suburban transition | 12 min71 subs | 47 min283 subs | 3 h 09 min1,132 subs |
| Bortle 5Suburban | 15 min90 subs | 1 h359 subs | 3 h 59 min1,433 subs |
| Bortle 6Bright suburban | 23 min136 subs | 1 h 30 min543 subs | 6 h 02 min2,172 subs |
| Bortle 7Suburban / urban transition | 37 min224 subs | 2 h 29 min896 subs | 9 h 57 min3,581 subs |
| Bortle 8City | 1 h361 subs | 4 h1,442 subs | 16 h5,767 subs |
| Bortle 9Inner city | 1 h 49 min652 subs | 7 h 15 min2,608 subs | 29 h10,431 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 1 h 58 min of run time. The Seestar S30’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 10000mAh 5V 3A6,000 mAh inside; a 10,000 mAh 5 V 3 A bank roughly doubles a session.
- Seestar S30 dew shieldHeater is built in; a cheap shield reduces dew and 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 S30 →
How it frames
The M108 is 8.7′ × 2.2′ across. The Seestar S30 frames 2.13° × 1.20° at 3.99″ per pixel, so it fills 12% of the frame’s short side.
Longer subs
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, which needs the TH10 head. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 39 min236 subs | 1 h359 subs | 2 h 29 min896 subs |
| 20 s | 32 min97 subs | 53 min159 subs | 2 h 22 min427 subs |
| 30 s | 30 min61 subs | 50 min101 subs | 2 h 20 min280 subs |
| 60 s | 30 min30 subs | 48 min49 subs | 2 h 18 min138 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 13 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 S30
Wider and lighter than the S50 with the same 2.9 µm pixels, so its per-pixel speed on extended targets is close despite the smaller aperture. Ideal for big nebulae.
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, which needs the TH10 head. Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters.
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 S30? 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: 1 h 26 min from Bortle 6
- M108 with the ZWO Seestar S50 Pro: 1 h 34 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 S30