How long to image the M28 with the ZWO Seestar S50 Pro
From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 7 min (42 subs of 10 s) for a solid image of the M28. A quick look takes 2 min; a showpiece 27 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 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 | 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 min91 subs |
| Bortle 4Rural / suburban transition | 2 min10 subs | 5 min30 subs | 17 min103 subs |
| Bortle 5Suburban | 2 min10 subs | 5 min31 subs | 20 min121 subs |
| Bortle 6Bright suburban | 2 min11 subs | 7 min42 subs | 27 min165 subs |
| Bortle 7Suburban / urban transition | 3 min16 subs | 10 min62 subs | 41 min248 subs |
| Bortle 8City | 4 min24 subs | 16 min95 subs | 1 h 03 min378 subs |
| Bortle 9Inner city | 7 min41 subs | 27 min164 subs | 1 h 49 min655 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 9 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 M28 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 | 5 min30 subs | 5 min31 subs | 10 min62 subs |
| 20 s | 10 min30 subs | 10 min30 subs | 10 min31 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 M28. 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 M28
A compact globular near Lambda Sagittarii, a degree from M22. Small and quick.
NGC 6626 · Globular cluster in Sagittarius · magnitude 6.8 · 11′ · best in Jun, Jul, Aug. 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 M28 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
- M28 with the ZWO Seestar S50: 6 min from Bortle 6
- M28 with the ZWO Seestar S30: 7 min from Bortle 6
- M28 with the ZWO Seestar S30 Pro: 7 min from Bortle 6
- M28 with the DwarfLab DWARF 3: 9 min from Bortle 6
- M28 with the DwarfLab DWARF mini: 8 min from Bortle 6
- M28 with the DwarfLab Draco: 15 min from Bortle 6
- M28 with the DwarfLab DWARF II: 20 min from Bortle 6
- M28 with the Vaonis Vespera II: 6 min from Bortle 6
- M28 with the Vaonis Vespera Pro: 14 min from Bortle 6
- M28 with the Vaonis Vespera 3: 6 min from Bortle 6
- M28 with the Vaonis Vespera Pro 2: 13 min from Bortle 6
- M28 with the Unistellar eQuinox 2: 5 min from Bortle 6
- M28 with the Unistellar Odyssey: 23 min from Bortle 6
- M28 with the Celestron Origin: 5 min from Bortle 6
- M28 with the Celestron Origin Mark II: 5 min from Bortle 6
All telescopes for the M28 · All targets for the Seestar S50 Pro