How long to image the M91 with the ZWO Seestar S30
From a Bortle 6 suburban sky, the ZWO Seestar S30 needs about 4 h 03 min (1,456 subs of 10 s) for a solid image of the M91. A quick look takes 1 h 01 min; a showpiece 16 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 40 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 | 22 min130 subs | 1 h 26 min519 subs | 5 h 45 min2,073 subs |
| Bortle 2Typical dark site | 23 min137 subs | 1 h 31 min547 subs | 6 h 05 min2,188 subs |
| Bortle 3Rural | 25 min151 subs | 1 h 40 min602 subs | 6 h 41 min2,406 subs |
| Bortle 4Rural / suburban transition | 31 min184 subs | 2 h 02 min734 subs | 8 h 09 min2,933 subs |
| Bortle 5Suburban | 39 min236 subs | 2 h 37 min943 subs | 10 h3,770 subs |
| Bortle 6Bright suburban | 1 h 01 min364 subs | 4 h 03 min1,456 subs | 16 h5,822 subs |
| Bortle 7Suburban / urban transition | 1 h 41 min609 subs | 6 h 45 min2,433 subs | 27 h9,731 subs |
| Bortle 8City | 2 h 45 min988 subs | 11 h3,951 subs | 44 h15,802 subs |
| Bortle 9Inner city | 4 h 59 min1,797 subs | 20 h7,188 subs | 80 h28,749 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 5 h 15 min of run time. The Seestar S30’s battery is rated at about 6 h, which is cutting it fine; take a power bank. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB-C power bank 10000mAh 5V 3A6,000 mAh inside; a 10,000 mAh 5 V 3 A bank roughly doubles a session.
- Seestar S30 dew shieldHeater is built in; a cheap shield reduces dew and stray light.
- 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 Seestar S30 →
How it frames
The M91 is 5.4′ × 4.3′ across. The Seestar S30 frames 2.13° × 1.20° at 3.99″ per pixel, so it fills 8% of the frame’s short side.
Longer subs
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, which needs the TH10 head. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 1 h 40 min602 subs | 2 h 37 min943 subs | 6 h 45 min2,433 subs |
| 20 s | 1 h 21 min244 subs | 2 h 18 min414 subs | 6 h 26 min1,159 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 |
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 13 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 ZWO Seestar S30
Wider and lighter than the S50 with the same 2.9 µm pixels, so its per-pixel speed on extended targets is close despite the smaller aperture. Ideal for big nebulae.
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, which needs the TH10 head. Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters.
Longer subs matter most under dark skies, where read noise rather than skyglow sets the floor. From a city the difference between 10 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 M91 from a Seestar S30? 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 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 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
All telescopes for the M91 · All targets for the Seestar S30