How long to image the M68 with the ZWO Seestar S50 Pro
From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 35 min (208 subs of 10 s) for a solid image of the M68. A quick look takes 9 min; a showpiece 2 h 18 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 13% 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 | 54 min325 subs |
| Bortle 2Typical dark site | 4 min22 subs | 14 min85 subs | 57 min340 subs |
| Bortle 3Rural | 4 min24 subs | 15 min93 subs | 1 h 02 min370 subs |
| Bortle 4Rural / suburban transition | 5 min28 subs | 18 min111 subs | 1 h 13 min441 subs |
| Bortle 5Suburban | 6 min35 subs | 23 min139 subs | 1 h 32 min554 subs |
| Bortle 6Bright suburban | 9 min52 subs | 35 min208 subs | 2 h 18 min831 subs |
| Bortle 7Suburban / urban transition | 14 min85 subs | 57 min340 subs | 3 h 46 min1,359 subs |
| Bortle 8City | 23 min137 subs | 1 h 31 min545 subs | 6 h 03 min2,179 subs |
| Bortle 9Inner city | 41 min246 subs | 2 h 44 min982 subs | 11 h3,927 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 45 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 M68 is 11′ across. The Seestar S50 Pro frames 2.45° × 1.38° at 2.30″ per pixel, so it fills 13% 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 | 15 min93 subs | 23 min139 subs | 57 min340 subs |
| 20 s | 13 min41 subs | 21 min64 subs | 55 min164 subs |
| 30 s | 15 min30 subs | 20 min41 subs | 54 min109 subs |
| 60 s | 30 min30 subs | 30 min30 subs | 53 min54 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 12 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 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 M68 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
- M68 with the ZWO Seestar S50: 32 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 Pro