How long to image the NGC 6939 with the ZWO Seestar S50
From a Bortle 6 suburban sky, the ZWO Seestar S50 needs about 6 min (34 subs of 10 s) for a solid image of the NGC 6939. A quick look takes 2 min; a showpiece 22 min. Under a Bortle 3 rural sky the solid figure drops to 5 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 16% 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 | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 2Typical dark site | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 3Rural | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 4Rural / suburban transition | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 5Suburban | 2 min10 subs | 5 min30 subs | 17 min102 subs |
| Bortle 6Bright suburban | 2 min10 subs | 6 min34 subs | 22 min133 subs |
| Bortle 7Suburban / urban transition | 2 min13 subs | 8 min49 subs | 32 min193 subs |
| Bortle 8City | 3 min18 subs | 12 min72 subs | 48 min286 subs |
| Bortle 9Inner city | 5 min31 subs | 20 min121 subs | 1 h 21 min484 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 7 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 NGC 6939 is 7′ across. The Seestar S50 frames 1.28° × 0.72° at 2.39″ per pixel, so it fills 16% 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 | 5 min30 subs | 5 min30 subs | 8 min49 subs |
| 20 s | 10 min30 subs | 10 min30 subs | 10 min30 subs |
| 30 s | 15 min30 subs | 15 min30 subs | 15 min30 subs |
| 60 s | 30 min30 subs | 30 min30 subs | 30 min30 subs |
Leave the dual-band filter out for the NGC 6939. It is an open cluster, 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 10 times slower.
About the NGC 6939
A rich, old open cluster that shares a frame with the Fireworks Galaxy. Cluster and galaxy in one shot.
Cep open cluster · Open cluster in Cepheus · magnitude 7.8 · 7′ · best in Jul, Aug, Sep. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
Open clusters are the easiest targets in the sky. The stars are bright, so the aim is colour and tight stars rather than depth: short subs, no filter, and stop before the bright members saturate. The nominal time here gets you clean star colour; more integration only adds background stars.
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 NGC 6939 from a Seestar S50? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- NGC 6939 with the ZWO Seestar S50 Pro: 6 min from Bortle 6
- NGC 6939 with the ZWO Seestar S30: 6 min from Bortle 6
- NGC 6939 with the ZWO Seestar S30 Pro: 6 min from Bortle 6
- NGC 6939 with the DwarfLab DWARF 3: 8 min from Bortle 6
- NGC 6939 with the DwarfLab DWARF mini: 8 min from Bortle 6
- NGC 6939 with the DwarfLab Draco: 15 min from Bortle 6
- NGC 6939 with the DwarfLab DWARF II: 18 min from Bortle 6
- NGC 6939 with the Vaonis Vespera II: 6 min from Bortle 6
- NGC 6939 with the Vaonis Vespera Pro: 12 min from Bortle 6
- NGC 6939 with the Vaonis Vespera 3: 5 min from Bortle 6
- NGC 6939 with the Vaonis Vespera Pro 2: 12 min from Bortle 6
- NGC 6939 with the Unistellar eQuinox 2: 4 min from Bortle 6
- NGC 6939 with the Unistellar Odyssey: 19 min from Bortle 6
- NGC 6939 with the Celestron Origin: 5 min from Bortle 6
- NGC 6939 with the Celestron Origin Mark II: 5 min from Bortle 6
All telescopes for the NGC 6939 · All targets for the Seestar S50