Astrophotography diagnosis of M 45_askar: Sensor tilt
Raw300s14 août 2026
The Doc examined this image of M 45_askar (raw, 300s). Estimated overall technical quality: 6/10. 1 defect found: Sensor tilt (severity 3/5).
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Info
- Cible
- M 45_askar
- Date
- 14 août 2026, 02:20
- Lune
- Gibbeuse décroissante 97.4% (sous l'horizon)
- Site
- Bortle 2 · rural typique (VIIRS)
- Position
- 3h46m58s · +24°03'21"
Excellent conditions: Bortle 2, and a 97% gibbous Moon but below the horizon and nearly 100° from the target, so no lunar pollution is possible. The measured sky background is flat and very dark, consistent with this sky. On a cluster like M45 this background SNR is ideal for pulling out the reflection nebulosity around the main stars: nothing limits the acquisition on the sky side, so you can stack many subs to bring out those faint blue veils.
Setup
- Type d'image
- Brut
- Télescope
- EQMod Mount
- Caméra
- ZWO ASI585MM Pro
- Exposition
- 300s
- Phase de lune
- Nouvelle lune (1 %)
- FOV
- 3.36°
Setup well matched to M45: at 190mm focal length your 3.36° field frames the whole cluster with comfortable margin, as the well-centered preview confirms. The 3.15″/px sampling is undersampled for a Bortle 2 sky but perfectly suited to a large open cluster where stellar sharpness isn't critical. Gain 150 and offset 15 on the ASI585MM are healthy, with no dark clipping (0%), and the sensor at -17.5°C is well cooled. The 300s exposure lifts the background adequately (e_margin 2.36), enough on this bright target.
The diagnosis in detail
This sub is technically strong in nearly every respect. The Moffat metrics confirm a perfectly round center (elong 1.13, exactly at the field floor) and a tight FWHM (~1.4 px), signs of controlled tracking and successful focus. The measured background is flat (0% radial profile, 0% plane gradient), which, under this moonless Bortle 2 sky, gives a high-quality stacking base.
The only real defect is sensor tilt. The asymmetry is neither radial nor symmetric: it runs along the horizontal axis, the right edge (FWHM 2.0/1.78/1.82 px) degrading far more than the left (1.5/1.63/1.43 px), with a 1.36 corner/center ratio. This is the signature of a slightly tilted focal plane, not field curvature or a backfocus error. On a cluster the effect stays visually subtle, but it will penalize the corners of a long integration.
The priority is therefore purely mechanical: adjust the parallelism of the optical train before stacking many subs, so this tilt isn't baked into the final master.
Priority actions
- Correct the sensor tilt (optical train parallelism / tilt-plate) by equalizing FWHM between the left and right edges before any long session
- Re-shoot a control aberration frame after adjustment to validate corner uniformity
- Take advantage of the moonless Bortle 2 sky to stack many subs and bring out M45's reflection nebulosity
