How long to image the M91 with the DwarfLab DWARF II
From a Bortle 6 suburban sky, the DwarfLab DWARF II needs about 13 h (3,164 subs of 15 s) for a solid image of the M91. A quick look takes 3 h 18 min; a showpiece 53 h. Under a Bortle 3 rural sky the solid figure drops to 6 h 35 min. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it fills 5% 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 | 1 h 29 min357 subs | 5 h 57 min1,428 subs | 24 h5,710 subs |
| Bortle 2Typical dark site | 1 h 33 min371 subs | 6 h 10 min1,481 subs | 25 h5,924 subs |
| Bortle 3Rural | 1 h 39 min396 subs | 6 h 35 min1,582 subs | 26 h6,327 subs |
| Bortle 4Rural / suburban transition | 1 h 54 min457 subs | 7 h 36 min1,826 subs | 30 h7,304 subs |
| Bortle 5Suburban | 2 h 18 min554 subs | 9 h 13 min2,214 subs | 37 h8,853 subs |
| Bortle 6Bright suburban | 3 h 18 min791 subs | 13 h3,164 subs | 53 h12,653 subs |
| Bortle 7Suburban / urban transition | 5 h 11 min1,244 subs | 21 h4,974 subs | 83 h19,893 subs |
| Bortle 8City | 8 h 06 min1,946 subs | 32 h7,784 subs | 130+ h31,135 subs |
| Bortle 9Inner city | 14 h3,445 subs | 57 h13,778 subs | 230+ h55,110 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 17 h of run time. That is more than one night: over 3 nights, about 5 h 43 min each. The DWARF II’s battery is rated at about 3 h, so each night needs an external power bank. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB-C PD power bank 10000mAh 18W18 W PD input; runs while charging.
- 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 II →
How it frames
The M91 is 5.4′ × 4.3′ across. The DWARF II frames 3.19° × 1.79° at 2.99″ per pixel, so it fills 5% of the frame’s short side.
About the M91
A faint barred spiral, the last Virgo cluster object Messier catalogued and the hardest of the group.
NGC 4548 · Galaxy in Coma Berenices · magnitude 10.2 · 5.4′ × 4.3′ · best in Mar, Apr, May. 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 II
The original pocket scope. 24 mm aperture and tiny 1.45 µm pixels mean it is slow per pixel, so bin or downsample and be patient.
15 s is the hard maximum in any mode; DwarfLab recommends EQ mode at 15 s. Two built-in filters, VIS and Astro (IR-pass); no dual-band. The optional UHC filter is no longer sold.
The DWARF II uses fixed 15 s subs, so the only levers you have are the sky you shoot from, the filter, and total time.
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 M91 from a DWARF II? Email it with your total time and sky to [email protected] and it goes here, credited.
Same target, other telescopes
- M91 with the ZWO Seestar S50: 3 h 51 min from Bortle 6
- M91 with the ZWO Seestar S50 Pro: 4 h 12 min from Bortle 6
- M91 with the ZWO Seestar S30: 4 h 03 min from Bortle 6
- M91 with the ZWO Seestar S30 Pro: 4 h 28 min from Bortle 6
- M91 with the DwarfLab DWARF 3: 5 h 35 min from Bortle 6
- M91 with the DwarfLab DWARF mini: 3 h 50 min from Bortle 6
- M91 with the DwarfLab Draco: 3 h 09 min from Bortle 6
- M91 with the Vaonis Vespera II: 3 h 51 min from Bortle 6
- M91 with the Vaonis Vespera Pro: 8 h 37 min from Bortle 6
- M91 with the Vaonis Vespera 3: 3 h 40 min from Bortle 6
- M91 with the Vaonis Vespera Pro 2: 8 h 13 min from Bortle 6
- M91 with the Unistellar eQuinox 2: 3 h 30 min from Bortle 6
- M91 with the Unistellar Odyssey: 16 h from Bortle 6
- M91 with the Celestron Origin: 1 h 01 min from Bortle 6
- M91 with the Celestron Origin Mark II: 1 h 24 min from Bortle 6