How long to image the M91 with the ZWO Seestar S50 Pro
From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 4 h 12 min (1,515 subs of 10 s) for a solid image of the M91. A quick look takes 1 h 03 min; a showpiece 17 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 39 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 7% 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 | 21 min126 subs | 1 h 24 min502 subs | 5 h 34 min2,005 subs |
| Bortle 2Typical dark site | 22 min134 subs | 1 h 29 min533 subs | 5 h 55 min2,130 subs |
| Bortle 3Rural | 25 min148 subs | 1 h 39 min592 subs | 6 h 34 min2,366 subs |
| Bortle 4Rural / suburban transition | 31 min184 subs | 2 h 02 min734 subs | 8 h 09 min2,936 subs |
| Bortle 5Suburban | 40 min241 subs | 2 h 40 min961 subs | 11 h3,841 subs |
| Bortle 6Bright suburban | 1 h 03 min379 subs | 4 h 12 min1,515 subs | 17 h6,060 subs |
| Bortle 7Suburban / urban transition | 1 h 47 min644 subs | 7 h 09 min2,573 subs | 29 h10,289 subs |
| Bortle 8City | 2 h 56 min1,054 subs | 12 h4,214 subs | 47 h16,855 subs |
| Bortle 9Inner city | 5 h 21 min1,929 subs | 21 h7,715 subs | 86 h30,858 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 5 h 28 min of run time. The Seestar S50 Pro’s battery (rated about 9 h) 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 30WAccepts 12 V 3 A PD, so a 30 W bank tops up the 10,000 mAh cell while imaging.
- Seestar S50 Pro dew shieldDew heater is built in; a shield adds margin on damp nights. Check the listing says S50 Pro.
- 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 S50 Pro →
How it frames
The M91 is 5.4′ × 4.3′ across. The Seestar S50 Pro frames 2.45° × 1.38° at 2.30″ per pixel, so it fills 7% of the frame’s short side.
Longer subs
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 1 h 39 min592 subs | 2 h 40 min961 subs | 7 h 09 min2,573 subs |
| 20 s | 1 h 23 min249 subs | 2 h 24 min434 subs | 6 h 53 min1,240 subs |
| 30 s | 1 h 18 min156 subs | 2 h 19 min279 subs | 6 h 48 min816 subs |
| 60 s | 1 h 13 min73 subs | 2 h 14 min135 subs | 6 h 43 min403 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 ZWO Seestar S50 Pro
The S50 optics at 260 mm on an 8 MP 1/1.2-inch sensor: a wider, taller field, four times the pixels, and 60 s subs in EQ mode. Per pixel it is a whisker slower than the S50 because of the longer focal length; the long subs make it the better dark-site scope.
10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters on the telephoto camera.
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 S50 Pro? 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 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 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 S50 Pro