How long to image the M91 with the Vaonis Vespera II
From a Bortle 6 suburban sky, the Vaonis Vespera II needs about 3 h 51 min (1,385 subs of 10 s) for a solid image of the M91. A quick look takes 58 min; a showpiece 15 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 29 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 6% 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 | 19 min112 subs | 1 h 15 min448 subs | 4 h 59 min1,792 subs |
| Bortle 2Typical dark site | 20 min120 subs | 1 h 19 min477 subs | 5 h 18 min1,907 subs |
| Bortle 3Rural | 22 min133 subs | 1 h 29 min532 subs | 5 h 54 min2,125 subs |
| Bortle 4Rural / suburban transition | 28 min166 subs | 1 h 50 min663 subs | 7 h 22 min2,652 subs |
| Bortle 5Suburban | 36 min218 subs | 2 h 25 min872 subs | 9 h 41 min3,488 subs |
| Bortle 6Bright suburban | 58 min347 subs | 3 h 51 min1,385 subs | 15 h5,540 subs |
| Bortle 7Suburban / urban transition | 1 h 38 min591 subs | 6 h 34 min2,363 subs | 26 h9,450 subs |
| Bortle 8City | 2 h 42 min971 subs | 11 h3,881 subs | 43 h15,521 subs |
| Bortle 9Inner city | 4 h 57 min1,780 subs | 20 h7,117 subs | 79 h28,467 subs |
Power and kit for this session
Allowing for slewing, focusing and rejected subs, that is about 5 h of run time. The Vespera II’s battery is rated at about 4 h, so the session needs an external power bank. (Bortle 6, solid image; we allow 30% over the integration time.)
- USB-C PD power bank 20000mAh 30WOnly four hours inside; Vaonis specifies a 30 W PD source. C-to-C cable not included.
- Vaonis Vespera hygrometer sensorTriggers the built-in demister automatically; the correct dew accessory for the II.
- 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 Vespera II →
How it frames
The M91 is 5.4′ × 4.3′ across. The Vespera II frames 2.55° × 1.44° at 2.39″ per pixel, so it fills 6% of the frame’s short side.
Longer subs
10 s subs by default; Expert Mode manual targets allow 1 to 30 s. Alt-azimuth only, no EQ mode. Time for a solid image by sub length:
| Sub length | Bortle 3 | Bortle 5 | Bortle 7 |
|---|---|---|---|
| 10 s | 1 h 29 min532 subs | 2 h 25 min872 subs | 6 h 34 min2,363 subs |
| 20 s | 1 h 15 min226 subs | 2 h 12 min397 subs | 6 h 20 min1,142 subs |
| 30 s | 1 h 11 min142 subs | 2 h 08 min256 subs | 6 h 16 min752 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 Vaonis Vespera II
A 50 mm quadruplet on an 8 MP sensor with Vaonis’s mosaic mode for wide fields. Same optics class as the Seestar S50, four times the pixels.
10 s subs by default; Expert Mode manual targets allow 1 to 30 s. Alt-azimuth only, no EQ mode. Optional clip-on Vaonis filters: dual-band, light-pollution (CLS) and solar; the app detects which is fitted.
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 Vespera II? 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 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