Astrophotography diagnosis of Crescent Nebula: Clipped stars, background chroma noise and 1 other
ProcessedNone38×180s31 mai 2026
The Doc examined this image of Crescent Nebula (processed, None, 38×180s). Estimated overall technical quality: 6/10. 3 defects found: Clipped stars (severity 2/5), Background chroma noise (severity 2/5), Green cast (severity 2/5).
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Info
- Cible
- Crescent Nebula
- Date
- 31 mai 2026, 00:00
- Position
- 20h12m12s · +38°21'00"
You imaged under a full Moon (100%), the worst context for a broadband RGB target with no filter. NGC 6888 emits mostly in Ha and OIII: its contrast collapses under a Moon-washed sky background, which explains the modest background SNR over 1h54 and the residual chroma noise. The result remains honorable because the target is intrinsically contrasty, but you lose a lot of faint signal. For this kind of object, either move the session away from the full Moon, or switch to a dual-band (L-eXtreme / L-Ultimate) that isolates Ha/OIII and makes the Moon nearly transparent.
Setup
- Type d'image
- Traitée
- Télescope
- Celestron EdgeHD 11"
- Caméra
- Player One Artemis-C Pro
- Filtre
- None
- Monture
- Celestron CGX-L
- Exposition
- 38×180s
- Phase de lune
- 100%
- Notes
- PixInsight, BlurXTerminator, Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
- FOV
- 42.0'
The EdgeHD 11 at 2800mm on the Artemis-C Pro gives a 0.70° field, perfectly suited to NGC 6888's size (~18'): the target is well framed, neither lost nor clipped. Sampling is fine (sub-pixel relative to seeing), consistent with the measured FWHM around 4.3 to 5 px. The weak point isn't the gear but the lack of a filter under a full Moon: on an emission target, a dual-band would be nearly essential here. The 180s subs are reasonable at this focal length, but the e_margin of 1.12 indicates a background barely lifted off the black point, a direct consequence of the lack of useful signal without a filter.
The diagnosis in detail
On star shape, the PSF panel is reassuring: homogeneous FWHM (4.2 to 5.0 px), a corner/center ratio of 1.04 and near-zero asymmetries rule out tilt, backfocus, coma or astigmatism. The center shows an elongation of 1.13 (field floor 1.03) with dispersed PA (35°), a sign of slight guiding/seeing spread rather than a clear directional defect: nothing to fix as a priority, BlurXTerminator has already tightened the stars well.
The real points of attention are colorimetric. The brightest stars have pure white cores (saturation/clipping), losing their B-V signature, and the sky background shows chromatic mottling (green/red/blue patches) typical of a modest SNR amplified by the stretch. On top of this is a greenish cast in the nebulosity, where Ha/OIII should appear red and turquoise.
The common cause is upstream: a full Moon with no filter, which caps the sky background and limits the faint signal collectable over 1h54. Processing then amplifies a poor background. The nebulosity itself is genuinely real (confirmed by the DSS), so it all comes down to color processing and future acquisition.
Priority actions
- Redo color calibration (SPCC) then apply SCNR to remove the green cast and recover Ha/OIII
- Protect highlights during the stretch and process stars separately to preserve their color
- Reduce background chroma noise after neutralization (chroma noise reduction)
- For future sessions: use a dual-band (L-eXtreme/L-Ultimate) or avoid the full Moon on this emission target
