How long to image the M68 with the ZWO Seestar S30
From a Bortle 6 suburban sky, the ZWO Seestar S30 needs about 33 min (199 subs of 10 s) for a solid image of the M68. A quick look takes 8 min; a showpiece 2 h 13 min. Under a Bortle 3 rural sky the solid figure drops to 15 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 15% 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 min21 subs | 14 min82 subs | 55 min328 subs |
| Bortle 2Typical dark site | 4 min22 subs | 14 min86 subs | 57 min342 subs |
| Bortle 3Rural | 4 min24 subs | 15 min93 subs | 1 h 01 min369 subs |
| Bortle 4Rural / suburban transition | 5 min28 subs | 18 min109 subs | 1 h 12 min435 subs |
| Bortle 5Suburban | 6 min34 subs | 22 min135 subs | 1 h 30 min539 subs |
| Bortle 6Bright suburban | 8 min50 subs | 33 min199 subs | 2 h 13 min796 subs |
| Bortle 7Suburban / urban transition | 13 min81 subs | 53 min321 subs | 3 h 34 min1,284 subs |
| Bortle 8City | 21 min128 subs | 1 h 25 min511 subs | 5 h 40 min2,042 subs |
| Bortle 9Inner city | 38 min229 subs | 2 h 32 min915 subs | 10 h3,658 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 43 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 M68 is 11′ across. The Seestar S30 frames 2.13° × 1.20° at 3.99″ per pixel, so it fills 15% 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 | 15 min93 subs | 22 min135 subs | 53 min321 subs |
| 20 s | 13 min39 subs | 20 min61 subs | 51 min154 subs |
| 30 s | 15 min30 subs | 19 min39 subs | 50 min101 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 13 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 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 M68 from a Seestar S30? 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: 32 min from Bortle 6
- M68 with the ZWO Seestar S50 Pro: 35 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 S30