How long to image the M21 with the ZWO Seestar S30
From a Bortle 6 suburban sky, the ZWO Seestar S30 needs about 5 min (30 subs of 10 s) for a solid image of the M21. A quick look takes 2 min; a showpiece 15 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 18% 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 min90 subs |
| Bortle 4Rural / suburban transition | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 5Suburban | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 6Bright suburban | 2 min10 subs | 5 min30 subs | 15 min90 subs |
| Bortle 7Suburban / urban transition | 2 min10 subs | 5 min30 subs | 18 min106 subs |
| Bortle 8City | 2 min10 subs | 6 min39 subs | 26 min156 subs |
| Bortle 9Inner city | 3 min17 subs | 11 min66 subs | 44 min263 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 7 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 M21 is 13′ across. The Seestar S30 frames 2.13° × 1.20° at 3.99″ per pixel, so it fills 18% 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 | 5 min30 subs | 5 min30 subs | 5 min30 subs |
| 20 s | 10 min30 subs | 10 min30 subs | 10 min30 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 M21. It is an open 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 7 times slower.
About the M21
A young open cluster beside the Trifid Nebula. Comes for free in a Trifid frame.
NGC 6531 · Open cluster in Sagittarius · magnitude 5.9 · 13′ · best in Jun, Jul, Aug. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
Open clusters are the easiest targets in the sky. The stars are bright, so the aim is colour and tight stars rather than depth: short subs, no filter, and stop before the bright members saturate. The nominal time here gets you clean star colour; more integration only adds background stars.
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 M21 from a Seestar S30? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- M21 with the ZWO Seestar S50: 5 min from Bortle 6
- M21 with the ZWO Seestar S50 Pro: 5 min from Bortle 6
- M21 with the ZWO Seestar S30 Pro: 5 min from Bortle 6
- M21 with the DwarfLab DWARF 3: 8 min from Bortle 6
- M21 with the DwarfLab DWARF mini: 8 min from Bortle 6
- M21 with the DwarfLab Draco: 15 min from Bortle 6
- M21 with the DwarfLab DWARF II: 9 min from Bortle 6
- M21 with the Vaonis Vespera II: 5 min from Bortle 6
- M21 with the Vaonis Vespera Pro: 7 min from Bortle 6
- M21 with the Vaonis Vespera 3: 5 min from Bortle 6
- M21 with the Vaonis Vespera Pro 2: 6 min from Bortle 6
- M21 with the Unistellar eQuinox 2: 2 min from Bortle 6
- M21 with the Unistellar Odyssey: 10 min from Bortle 6
- M21 with the Celestron Origin: 5 min from Bortle 6
- M21 with the Celestron Origin Mark II: 5 min from Bortle 6
All telescopes for the M21 · All targets for the Seestar S30