How long to image the M108 with the ZWO Seestar S50 Pro
From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 1 h 34 min (566 subs of 10 s) for a solid image of the M108. A quick look takes 24 min; a showpiece 6 h 17 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 11% 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 min51 subs | 33 min201 subs | 2 h 14 min804 subs |
| Bortle 2Typical dark site | 9 min54 subs | 35 min213 subs | 2 h 21 min849 subs |
| Bortle 3Rural | 10 min59 subs | 39 min234 subs | 2 h 36 min934 subs |
| Bortle 4Rural / suburban transition | 12 min72 subs | 47 min285 subs | 3 h 10 min1,139 subs |
| Bortle 5Suburban | 15 min92 subs | 1 h 01 min367 subs | 4 h 04 min1,465 subs |
| Bortle 6Bright suburban | 24 min142 subs | 1 h 34 min566 subs | 6 h 17 min2,264 subs |
| Bortle 7Suburban / urban transition | 39 min237 subs | 2 h 38 min947 subs | 11 h3,788 subs |
| Bortle 8City | 1 h 04 min385 subs | 4 h 16 min1,539 subs | 17 h6,153 subs |
| Bortle 9Inner city | 1 h 57 min700 subs | 7 h 47 min2,800 subs | 31 h11,197 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 2 h 03 min of run time. The Seestar S50 Pro’s battery (rated about 9 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 PD power bank 20000mAh 30WAccepts 12 V 3 A PD, so a 30 W bank tops up the 10,000 mAh cell while imaging.
- Seestar S50 Pro dew shieldDew heater is built in; a shield adds margin on damp nights. Check the listing says S50 Pro.
- 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 Pro →
How it frames
The M108 is 8.7′ × 2.2′ across. The Seestar S50 Pro frames 2.45° × 1.38° at 2.30″ per pixel, so it fills 11% of the frame’s short side.
Longer subs
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 39 min234 subs | 1 h 01 min367 subs | 2 h 38 min947 subs |
| 20 s | 33 min100 subs | 55 min167 subs | 2 h 32 min457 subs |
| 30 s | 31 min63 subs | 54 min108 subs | 2 h 30 min301 subs |
| 60 s | 30 min30 subs | 52 min52 subs | 2 h 28 min149 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 Pro
The S50 optics at 260 mm on an 8 MP 1/1.2-inch sensor: a wider, taller field, four times the pixels, and 60 s subs in EQ mode. Per pixel it is a whisker slower than the S50 because of the longer focal length; the long subs make it the better dark-site scope.
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters on the telephoto camera.
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 Pro? 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 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 Pro