How long to image the M55 with the ZWO Seestar S50 Pro
From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 15 min (91 subs of 10 s) for a solid image of the M55. A quick look takes 4 min; a showpiece 1 h. Under a Bortle 3 rural sky the solid figure drops to 7 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 23% 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 | 6 min39 subs | 25 min153 subs |
| Bortle 2Typical dark site | 2 min10 subs | 7 min40 subs | 26 min159 subs |
| Bortle 3Rural | 2 min11 subs | 7 min43 subs | 28 min171 subs |
| Bortle 4Rural / suburban transition | 2 min13 subs | 8 min51 subs | 33 min201 subs |
| Bortle 5Suburban | 3 min16 subs | 10 min62 subs | 41 min247 subs |
| Bortle 6Bright suburban | 4 min23 subs | 15 min91 subs | 1 h361 subs |
| Bortle 7Suburban / urban transition | 6 min37 subs | 24 min145 subs | 1 h 36 min579 subs |
| Bortle 8City | 10 min58 subs | 38 min229 subs | 2 h 33 min916 subs |
| Bortle 9Inner city | 17 min103 subs | 1 h 08 min409 subs | 4 h 33 min1,636 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 20 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.
Affiliate links: the retailer pays us a small commission and you pay the same price. These are search links to the named product, not endorsements of a particular listing; check the listing matches your model.
Everything that fits the Seestar S50 Pro →
How it frames
The M55 is 19′ across. The Seestar S50 Pro frames 2.45° × 1.38° at 2.30″ per pixel, so it fills 23% 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 | 7 min43 subs | 10 min62 subs | 24 min145 subs |
| 20 s | 10 min30 subs | 10 min30 subs | 23 min70 subs |
| 30 s | 15 min30 subs | 15 min30 subs | 23 min47 subs |
| 60 s | 30 min30 subs | 30 min30 subs | 30 min30 subs |
Leave the dual-band filter out for the M55. 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 M55
A large, loose globular that resolves easily but sits low for northern imagers.
NGC 6809 · Globular cluster in Sagittarius · magnitude 6.3 · 19′ · 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 M55 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
- M55 with the ZWO Seestar S50: 14 min from Bortle 6
- M55 with the ZWO Seestar S30: 14 min from Bortle 6
- M55 with the ZWO Seestar S30 Pro: 16 min from Bortle 6
- M55 with the DwarfLab DWARF 3: 20 min from Bortle 6
- M55 with the DwarfLab DWARF mini: 14 min from Bortle 6
- M55 with the DwarfLab Draco: 15 min from Bortle 6
- M55 with the DwarfLab DWARF II: 46 min from Bortle 6
- M55 with the Vaonis Vespera II: 14 min from Bortle 6
- M55 with the Vaonis Vespera Pro: 31 min from Bortle 6
- M55 with the Vaonis Vespera 3: 13 min from Bortle 6
- M55 with the Vaonis Vespera Pro 2: 29 min from Bortle 6
- M55 with the Unistellar eQuinox 2: 12 min from Bortle 6
- M55 with the Unistellar Odyssey: 54 min from Bortle 6
- M55 with the Celestron Origin: 5 min from Bortle 6
- M55 with the Celestron Origin Mark II: 5 min from Bortle 6
All telescopes for the M55 · All targets for the Seestar S50 Pro