How long to image the M21 with the DwarfLab DWARF 3
From a Bortle 6 suburban sky, the DwarfLab DWARF 3 needs about 8 min (30 subs of 15 s) for a solid image of the M21. A quick look takes 3 min; a showpiece 23 min. Under a Bortle 3 rural sky the solid figure drops to 8 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 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 2Typical dark site | 3 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 3Rural | 3 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 4Rural / suburban transition | 3 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 5Suburban | 3 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 6Bright suburban | 3 min10 subs | 8 min30 subs | 23 min90 subs |
| Bortle 7Suburban / urban transition | 3 min10 subs | 8 min30 subs | 26 min104 subs |
| Bortle 8City | 3 min10 subs | 10 min39 subs | 39 min156 subs |
| Bortle 9Inner city | 4 min17 subs | 17 min67 subs | 1 h 06 min266 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 10 min of run time. The DWARF 3’s battery (rated about 5 h 30 min) 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 18W18 W PD input and it runs while charging; needs a C-to-C cable for PD.
- 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 DWARF 3 →
How it frames
The M21 is 13′ across. The DWARF 3 frames 2.93° × 1.65° at 2.75″ per pixel, so it fills 13% of the frame’s short side.
Longer subs
Auto mode caps subs at 15 s; 30 s and longer need EQ mode, and 60 s is the ceiling on the tele camera. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 15 s | 8 min30 subs | 8 min30 subs | 8 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 5 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 DwarfLab DWARF 3
Small pixels on a 35 mm f/4.3 lens give a sharper image than its aperture suggests, and EQ mode with 60 s subs makes it the read-noise winner in dark skies.
Auto mode caps subs at 15 s; 30 s and longer need EQ mode, and 60 s is the ceiling on the tele camera. Three built-in filters: VIS, Astro (430 to 690 nm) and dual-band Hα 656 nm / OIII 501 nm.
Longer subs matter most under dark skies, where read noise rather than skyglow sets the floor. From a city the difference between 15 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 DWARF 3? 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: 5 min from Bortle 6
- M21 with the ZWO Seestar S30 Pro: 5 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