How long to image the M91 with the DwarfLab DWARF mini
From a Bortle 6 suburban sky, the DwarfLab DWARF mini needs about 3 h 50 min (920 subs of 15 s) for a solid image of the M91. A quick look takes 57 min; a showpiece 15 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 27 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 8% 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 | 18 min74 subs | 1 h 14 min295 subs | 4 h 54 min1,178 subs |
| Bortle 2Typical dark site | 20 min79 subs | 1 h 18 min314 subs | 5 h 13 min1,254 subs |
| Bortle 3Rural | 22 min88 subs | 1 h 27 min350 subs | 5 h 50 min1,399 subs |
| Bortle 4Rural / suburban transition | 27 min110 subs | 1 h 49 min438 subs | 7 h 18 min1,751 subs |
| Bortle 5Suburban | 36 min145 subs | 2 h 24 min578 subs | 9 h 37 min2,309 subs |
| Bortle 6Bright suburban | 57 min230 subs | 3 h 50 min920 subs | 15 h3,677 subs |
| Bortle 7Suburban / urban transition | 1 h 38 min393 subs | 6 h 33 min1,571 subs | 26 h6,283 subs |
| Bortle 8City | 2 h 41 min646 subs | 11 h2,583 subs | 43 h10,330 subs |
| Bortle 9Inner city | 4 h 56 min1,186 subs | 20 h4,741 subs | 79 h18,961 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 4 h 59 min of run time. The DWARF mini’s battery is rated at about 3 h 30 min, so the session needs an external power bank. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB-C PD power bank 10000mAh 20W7,000 mAh inside, about four hours; a PD bank extends the night.
- 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 mini →
How it frames
The M91 is 5.4′ × 4.3′ across. The DWARF mini frames 2.13° × 1.20° at 3.99″ per pixel, so it fills 8% of the frame’s short side.
Longer subs
Auto mode caps subs at 15 s; 30 s and longer need EQ mode, and 90 s is the ceiling. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 15 s | 1 h 27 min350 subs | 2 h 24 min578 subs | 6 h 33 min1,571 subs |
| 30 s | 1 h 15 min150 subs | 2 h 11 min263 subs | 6 h 20 min760 subs |
| 60 s | 1 h 08 min69 subs | 2 h 05 min126 subs | 6 h 14 min374 subs |
| 90 s | 1 h 06 min45 subs | 2 h 03 min82 subs | 6 h 11 min248 subs |
Leave the dual-band filter out for the M91. 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 12 times slower.
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 mini
DwarfLab’s answer to the Seestar S30: same sensor and focal length, big pixels, dual-band filter built in, and 90 s subs in EQ mode. Sensitivity over megapixels.
Auto mode caps subs at 15 s; 30 s and longer need EQ mode, and 90 s is the ceiling. Built-in Astro (430 to 690 nm), dark-frame and dual-band Hα / OIII filters; no plain VIS filter on the tele camera.
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 90 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 M91 from a DWARF mini? 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 Draco: 3 h 09 min from Bortle 6
- M91 with the DwarfLab DWARF II: 13 h 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