How long to image the M91 with the Celestron Origin
From a Bortle 6 suburban sky, the Celestron Origin needs about 1 h 01 min (367 subs of 10 s) for a solid image of the M91. A quick look takes 15 min; a showpiece 4 h 04 min. Under a Bortle 3 rural sky the solid figure drops to 21 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 11% 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 | 4 min26 subs | 17 min101 subs | 1 h 07 min402 subs |
| Bortle 2Typical dark site | 5 min28 subs | 18 min109 subs | 1 h 12 min435 subs |
| Bortle 3Rural | 5 min32 subs | 21 min125 subs | 1 h 23 min497 subs |
| Bortle 4Rural / suburban transition | 7 min41 subs | 27 min162 subs | 1 h 48 min646 subs |
| Bortle 5Suburban | 9 min56 subs | 37 min221 subs | 2 h 27 min884 subs |
| Bortle 6Bright suburban | 15 min92 subs | 1 h 01 min367 subs | 4 h 04 min1,466 subs |
| Bortle 7Suburban / urban transition | 27 min161 subs | 1 h 47 min644 subs | 7 h 09 min2,575 subs |
| Bortle 8City | 45 min269 subs | 2 h 59 min1,075 subs | 12 h4,297 subs |
| Bortle 9Inner city | 1 h 23 min499 subs | 5 h 32 min1,993 subs | 22 h7,970 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 1 h 19 min of run time. The Origin’s battery (rated about 6 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.)
- portable power station 300W AC outletRuns the supplied 12 V adapter in the field; the dew shield and heater are already built in.
- red LED head torchRed light keeps your night vision; white light costs you twenty minutes of dark adaptation every time.
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How it frames
The M91 is 5.4′ × 4.3′ across. The Origin frames 1.27° × 0.85° at 1.48″ per pixel, so it fills 11% of the frame’s short side.
Longer subs
10 s auto subs (15 s with the Nebula Filter); manual 1 to 30 s on the alt-az mount, longer only with the EQ wedge, and 60 s and up needs the StarSense Autoguider too. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 21 min125 subs | 37 min221 subs | 1 h 47 min644 subs |
| 15 s | 19 min78 subs | 35 min142 subs | 1 h 46 min424 subs |
| 30 s | 18 min36 subs | 34 min68 subs | 1 h 44 min209 subs |
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 Celestron Origin
The first-generation Origin: a 6-inch f/2.2 RASA in a smart package and the fastest system here by a wide margin. Superseded by the Mark II in late 2025 and no longer sold new; Celestron offers a camera upgrade to the Mark II sensor.
10 s auto subs (15 s with the Nebula Filter); manual 1 to 30 s on the alt-az mount, longer only with the EQ wedge, and 60 s and up needs the StarSense Autoguider too. Filter drawer takes 1.25-inch or 2-inch filters, including dual-band; Celestron sells an optional Nebula Filter.
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 30 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 Origin? 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 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 Mark II: 1 h 24 min from Bortle 6