How long to image the M68 with the ZWO Seestar S50
From a Bortle 6 suburban sky, the ZWO Seestar S50 needs about 32 min (190 subs of 10 s) for a solid image of the M68. A quick look takes 8 min; a showpiece 2 h 07 min. Under a Bortle 3 rural sky the solid figure drops to 14 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 26% 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 | 3 min19 subs | 12 min73 subs | 49 min292 subs |
| Bortle 2Typical dark site | 3 min20 subs | 13 min77 subs | 51 min307 subs |
| Bortle 3Rural | 3 min21 subs | 14 min84 subs | 56 min334 subs |
| Bortle 4Rural / suburban transition | 4 min25 subs | 17 min100 subs | 1 h 07 min400 subs |
| Bortle 5Suburban | 5 min32 subs | 21 min126 subs | 1 h 24 min504 subs |
| Bortle 6Bright suburban | 8 min48 subs | 32 min190 subs | 2 h 07 min760 subs |
| Bortle 7Suburban / urban transition | 13 min79 subs | 52 min313 subs | 3 h 28 min1,249 subs |
| Bortle 8City | 21 min126 subs | 1 h 24 min502 subs | 5 h 34 min2,006 subs |
| Bortle 9Inner city | 38 min227 subs | 2 h 31 min906 subs | 10 h3,623 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 41 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 M68 is 11′ across. The Seestar S50 frames 1.28° × 0.72° at 2.39″ per pixel, so it fills 26% 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 | 14 min84 subs | 21 min126 subs | 52 min313 subs |
| 20 s | 12 min37 subs | 19 min58 subs | 50 min152 subs |
| 30 s | 15 min30 subs | 19 min38 subs | 50 min100 subs |
| 60 s | 30 min30 subs | 30 min30 subs | 49 min50 subs |
Leave the dual-band filter out for the M68. It is a globular 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 11 times slower.
About the M68
A loose globular in Hydra, low from the UK but easy from lower latitudes.
NGC 4590 · Globular cluster in Hydra · magnitude 7.8 · 11′ · best in Mar, Apr, May. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
Globular clusters are point sources packed tight, so they come quickly: the core resolves into stars within a handful of minutes on any smart scope. More time deepens the halo and pulls in the faint outer members. Use short subs so the core does not saturate into a white blob, and no filter.
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 M68 from a Seestar S50? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- M68 with the ZWO Seestar S50 Pro: 35 min from Bortle 6
- M68 with the ZWO Seestar S30: 33 min from Bortle 6
- M68 with the ZWO Seestar S30 Pro: 37 min from Bortle 6
- M68 with the DwarfLab DWARF 3: 46 min from Bortle 6
- M68 with the DwarfLab DWARF mini: 32 min from Bortle 6
- M68 with the DwarfLab Draco: 26 min from Bortle 6
- M68 with the DwarfLab DWARF II: 1 h 47 min from Bortle 6
- M68 with the Vaonis Vespera II: 32 min from Bortle 6
- M68 with the Vaonis Vespera Pro: 1 h 11 min from Bortle 6
- M68 with the Vaonis Vespera 3: 30 min from Bortle 6
- M68 with the Vaonis Vespera Pro 2: 1 h 07 min from Bortle 6
- M68 with the Unistellar eQuinox 2: 28 min from Bortle 6
- M68 with the Unistellar Odyssey: 2 h 08 min from Bortle 6
- M68 with the Celestron Origin: 8 min from Bortle 6
- M68 with the Celestron Origin Mark II: 12 min from Bortle 6
All telescopes for the M68 · All targets for the Seestar S50