How long to image the NGC 891 with the DwarfLab DWARF 3
From a Bortle 6 suburban sky, the DwarfLab DWARF 3 needs about 3 h 58 min (954 subs of 15 s) for a solid image of the NGC 891. A quick look takes 1 h; a showpiece 16 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 20 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 14% 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 | 16 min65 subs | 1 h 04 min257 subs | 4 h 16 min1,026 subs |
| Bortle 2Typical dark site | 17 min70 subs | 1 h 09 min278 subs | 4 h 38 min1,112 subs |
| Bortle 3Rural | 20 min80 subs | 1 h 20 min319 subs | 5 h 18 min1,274 subs |
| Bortle 4Rural / suburban transition | 26 min105 subs | 1 h 44 min417 subs | 6 h 56 min1,666 subs |
| Bortle 5Suburban | 36 min143 subs | 2 h 23 min572 subs | 9 h 32 min2,287 subs |
| Bortle 6Bright suburban | 1 h239 subs | 3 h 58 min954 subs | 16 h3,813 subs |
| Bortle 7Suburban / urban transition | 1 h 45 min420 subs | 7 h1,680 subs | 28 h6,719 subs |
| Bortle 8City | 2 h 55 min702 subs | 12 h2,808 subs | 47 h11,231 subs |
| Bortle 9Inner city | 5 h 26 min1,304 subs | 22 h5,214 subs | 87 h20,854 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 5 h 10 min of run time. The DWARF 3’s battery is rated at about 5 h 30 min, less near freezing, so the session needs an external power bank. (Bortle 6, solid image; we allow 30% over the integration time, and 30% off the battery for a November night.)
- 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.
- hand warmers long lastingAir-activated packs give hours of heat for the person; winter nights are long and you spend most of them standing still.
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Everything that fits the DWARF 3 →
How it frames
The NGC 891 is 13.5′ × 2.5′ across. The DWARF 3 frames 2.93° × 1.65° at 2.75″ per pixel, so it fills 14% 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 | 1 h 20 min319 subs | 2 h 23 min572 subs | 7 h1,680 subs |
| 30 s | 1 h 14 min149 subs | 2 h 17 min275 subs | 6 h 54 min829 subs |
| 60 s | 1 h 11 min72 subs | 2 h 15 min135 subs | 6 h 52 min412 subs |
Leave the dual-band filter out for the NGC 891. 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 NGC 891
A perfect edge-on spiral split by a dust lane. Faint per pixel; a dark sky or a long stack shows the lane.
Caldwell 23, Silver Sliver · Galaxy in Andromeda · magnitude 10.0 · 13.5′ × 2.5′ · best in Oct, Nov, Dec. 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 NGC 891 from a DWARF 3? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- NGC 891 with the ZWO Seestar S50: 2 h 44 min from Bortle 6
- NGC 891 with the ZWO Seestar S50 Pro: 3 h from Bortle 6
- NGC 891 with the ZWO Seestar S30: 2 h 53 min from Bortle 6
- NGC 891 with the ZWO Seestar S30 Pro: 3 h 10 min from Bortle 6
- NGC 891 with the DwarfLab DWARF mini: 2 h 43 min from Bortle 6
- NGC 891 with the DwarfLab Draco: 2 h 15 min from Bortle 6
- NGC 891 with the DwarfLab DWARF II: 9 h 24 min from Bortle 6
- NGC 891 with the Vaonis Vespera II: 2 h 44 min from Bortle 6
- NGC 891 with the Vaonis Vespera Pro: 6 h 08 min from Bortle 6
- NGC 891 with the Vaonis Vespera 3: 2 h 37 min from Bortle 6
- NGC 891 with the Vaonis Vespera Pro 2: 5 h 51 min from Bortle 6
- NGC 891 with the Unistellar eQuinox 2: 2 h 30 min from Bortle 6
- NGC 891 with the Unistellar Odyssey: 12 h from Bortle 6
- NGC 891 with the Celestron Origin: 43 min from Bortle 6
- NGC 891 with the Celestron Origin Mark II: 59 min from Bortle 6
All telescopes for the NGC 891 · All targets for the DWARF 3