Astrophotography diagnosis of N?buleuse du Croissant: Sensor tilt
RawUV/IR30s08 août 2026
The Doc examined this image of N?buleuse du Croissant (raw, UV/IR, 30s). Estimated overall technical quality: 6/10. 1 defect found: Sensor tilt (severity 3/5).
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Info
- Cible
- N?buleuse du Croissant
- Date
- 08 août 2026, 22:13
- Lune
- Gibbeuse croissante 80.8% (sous l'horizon)
- Site
- Bortle 3 · rural peu pollué (saisi)
- Position
- 20h12m09s · +38°21'25"
Reported Bortle 3 sky and the Moon below the horizon (80.8% gibbous but 104.7 deg away and not risen): near-ideal conditions, no lunar pollution possible and a dark background that explains the good nebula contrast at just 30s. On an emission target like NGC 6888 this sky allows long exposures (120-300s) without saturating the background and recovers the faint OIII halo. Nothing to fix on conditions, this is the time to pile on integration.
Setup
- Type d'image
- Brut
- Télescope
- Bresser 200/1140
- Caméra
- SVBONY SV405CC
- Filtre
- UV/IR
- Monture
- EQ6-R
- Exposition
- 30s
- Phase de lune
- Dernier croissant (20 %)
- Seeing
- 3
- FOV
- 57.9'
Setup is well matched to the target: at 1140 mm the 0.96 deg field frames the Crescent nicely, centered with comfortable margins, and 0.84 arcsec/px sampling is slightly oversampled for 3 arcsec seeing but perfectly usable on this f/5.6 Newtonian. Healthy settings: gain 122, offset 15 with no dark clipping (0.01%), sensor at -4.7C. The 30s exposure stays short: the background is not yet sky-limited (e_margin 2.25), so you can push to 120-180s given the absent Moon to lift the signal above read noise.
The diagnosis in detail
The diagnosis is dominated by a single instrumental issue, cleanly identified by the PSF panel. The field center is excellent (elong 1.16, right on the OSC debayered floor) and both upper corners stay round, which rules out field curvature or symmetric backfocus. The two lower corners, however, blow up (BL elong 1.67, BR 1.47) with diverging PA: the classic signature of a sensor plane tilted relative to the focal plane. The corner/center ratio of 1.37 and vertical asymmetry of 0.227 (vs 0.043 horizontal) confirm the tilt runs along the sensor's vertical axis.
The rest of the frame is healthy for a raw sub. Tracking is good: no directional elongation at center and no coherent field-wide drift, the EQ6-R holds 30s without trouble. The background map shows a mild plane gradient (8%) but consistent with the dense Milky Way in this Cygnus region (DSS check: the field truly is packed with stars and diffuse nebulosity), so not retained as a defect at this pre-calibration stage.
Priority: fix the tilt mechanically before collecting the series, otherwise every sub will inherit these stretched corners that stacking cannot correct.
Priority actions
- Correct sensor tilt: evenly tighten the imaging train then adjust a push-pull plate until the four corners' FWHM matches
- Increase sub exposure to 120-180s (no Moon, Bortle 3) to become sky-limited and improve SNR
- Rerun an aberration inspector check after adjustment to validate field uniformity before stacking the session
