How long to image the M91 with the Unistellar Odyssey
From a Bortle 6 suburban sky, the Unistellar Odyssey needs about 16 h (14,570 subs of 4 s) for a solid image of the M91. A quick look takes 4 h 03 min; a showpiece 65 h. Under a Bortle 3 rural sky the solid figure drops to 11 h. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it fills 16% 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 | 2 h 34 min2,315 subs | 10 h9,257 subs | 41 h37,025 subs |
| Bortle 2Typical dark site | 2 h 37 min2,355 subs | 10 h9,420 subs | 42 h37,678 subs |
| Bortle 3Rural | 2 h 42 min2,433 subs | 11 h9,729 subs | 43 h38,913 subs |
| Bortle 4Rural / suburban transition | 2 h 55 min2,619 subs | 12 h10,476 subs | 47 h41,904 subs |
| Bortle 5Suburban | 3 h 14 min2,916 subs | 13 h11,662 subs | 52 h46,646 subs |
| Bortle 6Bright suburban | 4 h 03 min3,643 subs | 16 h14,570 subs | 65 h58,278 subs |
| Bortle 7Suburban / urban transition | 5 h 35 min5,028 subs | 22 h20,111 subs | 89 h80,444 subs |
| Bortle 8City | 7 h 59 min7,179 subs | 32 h28,715 subs | 130+ h114,859 subs |
| Bortle 9Inner city | 13 h11,766 subs | 52 h47,064 subs | 210+ h188,256 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 21 h of run time. That is more than one night: over 4 nights, about 5 h 16 min each. The Odyssey’s battery is rated at about 5 h, so each night needs an external power bank. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB power bank 20000mAh 5V 2.4AUnistellar says plain 5 V 2.4 A and warns against fast or adaptive chargers.
- 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 Odyssey →
How it frames
The M91 is 5.4′ × 4.3′ across. The Odyssey frames 0.75° × 0.56° at 0.93″ per pixel, so it fills 16% 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 Unistellar Odyssey
Unistellar’s compact 85 mm with autofocus and tiny pixels. Sharp, but the fine sampling costs time on faint extended targets. The Odyssey Pro is the same telescope with an eyepiece added, so these figures apply to both.
Fixed 4 s subs in Enhanced Vision; they cannot be changed. No filter option; Unistellar says third-party filters cannot be fitted to the Odyssey.
The Odyssey uses fixed 4 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 Odyssey? 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 Celestron Origin: 1 h 01 min from Bortle 6
- M91 with the Celestron Origin Mark II: 1 h 24 min from Bortle 6