How long to image the M108 with the DwarfLab DWARF 3
From a Bortle 6 suburban sky, the DwarfLab DWARF 3 needs about 2 h 06 min (503 subs of 15 s) for a solid image of the M108. A quick look takes 31 min; a showpiece 8 h 22 min. Under a Bortle 3 rural sky the solid figure drops to 46 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 9% 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 | 10 min39 subs | 39 min155 subs | 2 h 35 min619 subs |
| Bortle 2Typical dark site | 10 min42 subs | 41 min166 subs | 2 h 45 min662 subs |
| Bortle 3Rural | 12 min47 subs | 46 min186 subs | 3 h 05 min742 subs |
| Bortle 4Rural / suburban transition | 15 min59 subs | 59 min235 subs | 3 h 54 min938 subs |
| Bortle 5Suburban | 19 min78 subs | 1 h 18 min312 subs | 5 h 12 min1,248 subs |
| Bortle 6Bright suburban | 31 min126 subs | 2 h 06 min503 subs | 8 h 22 min2,010 subs |
| Bortle 7Suburban / urban transition | 54 min217 subs | 3 h 36 min865 subs | 14 h3,460 subs |
| Bortle 8City | 1 h 29 min357 subs | 5 h 57 min1,428 subs | 24 h5,712 subs |
| Bortle 9Inner city | 2 h 44 min658 subs | 11 h2,629 subs | 44 h10,514 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 2 h 43 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 M108 is 8.7′ × 2.2′ across. The DWARF 3 frames 2.93° × 1.65° at 2.75″ per pixel, so it fills 9% 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 | 46 min186 subs | 1 h 18 min312 subs | 3 h 36 min865 subs |
| 30 s | 44 min88 subs | 1 h 15 min151 subs | 3 h 33 min427 subs |
| 60 s | 42 min43 subs | 1 h 14 min74 subs | 3 h 32 min213 subs |
Leave the dual-band filter out for the M108. It is a galaxy, 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 10 times slower.
About the M108
An edge-on spiral with a mottled, dusty disc, a degree from the Owl Nebula. Frame both together.
NGC 3556 · Galaxy in Ursa Major · magnitude 10.0 · 8.7′ × 2.2′ · best in Feb, Mar, Apr. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.
Galaxies are broadband targets: their light is mostly starlight spread across the spectrum, so a dual-band filter throws most of it away and makes things slower, not faster. Shoot without a filter (or with a plain UV/IR cut) and rely on total time and a dark sky. The core of a galaxy is usually many times brighter than its outer disc, so a short stack shows the nucleus long before the arms; the figures here are for the arms and disc, not the core.
Field rotation in Alt-Az mode crops the edges of the stack over a long session, so on a big galaxy re-centre now and then or use EQ mode where your scope supports it.
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 M108 from a DWARF 3? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- M108 with the ZWO Seestar S50: 1 h 26 min from Bortle 6
- M108 with the ZWO Seestar S50 Pro: 1 h 34 min from Bortle 6
- M108 with the ZWO Seestar S30: 1 h 30 min from Bortle 6
- M108 with the ZWO Seestar S30 Pro: 1 h 40 min from Bortle 6
- M108 with the DwarfLab DWARF mini: 1 h 26 min from Bortle 6
- M108 with the DwarfLab Draco: 1 h 11 min from Bortle 6
- M108 with the DwarfLab DWARF II: 4 h 54 min from Bortle 6
- M108 with the Vaonis Vespera II: 1 h 26 min from Bortle 6
- M108 with the Vaonis Vespera Pro: 3 h 13 min from Bortle 6
- M108 with the Vaonis Vespera 3: 1 h 22 min from Bortle 6
- M108 with the Vaonis Vespera Pro 2: 3 h 04 min from Bortle 6
- M108 with the Unistellar eQuinox 2: 1 h 18 min from Bortle 6
- M108 with the Unistellar Odyssey: 5 h 57 min from Bortle 6
- M108 with the Celestron Origin: 23 min from Bortle 6
- M108 with the Celestron Origin Mark II: 31 min from Bortle 6