Astrophotography diagnosis of Flat bleu 27juin26: no defect detected
ProcessedHa3s27 juin 2026
The Doc examined this image of Flat bleu 27juin26 (processed, Ha, 3s). No significant defect was found on this acquisition.
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Info
- Cible
- Flat bleu 27juin26
- Date
- 27 juin 2026, 14:10
- Lune
- Croissant décroissant 5.1% (sous l'horizon)
- Site
- Bortle 1 · ciel pristine (VIIRS)
- Position
- 7h52m42s · +2°31'54"
Not enough information for an opinion from the Doc.
Setup
- Type d'image
- Traitée
- Télescope
- ASKARFR600MM /0,70
- Caméra
- QHY294PROM
- Filtre
- Ha
- Exposition
- 3s
- Phase de lune
- Gibbeuse croissante (94 %)
- FOV
- 2.62°
On this ASKAR FR600 (f/4, 421mm) with the QHY294M Pro in Ha, the flat is correctly dosed: median at 0.33 of the dynamic range, right in the recommended window (30-50%), no dark clipping (offset 0 with no truncation of the left tail, and the median is high so no risk here) and no saturated pixels. The sensor is cooled to -10.2°C, healthy. Gain 1600 is the conversion-gain switch point of the 294, a usual value, with no impact on a flat. The real issue is light distribution: the radial model is weak (R²=0.21) while the plane model dominates (R²=0.6, dominance 2.82), a sign of uneven illumination of the flat rather than plain optical vignetting. Make your light source more uniform.
The diagnosis in detail
The analyzed frame is a Ha calibration flat, not a sky exposure: the aberration inspector board returns n=0 in all nine zones, which is expected behavior (no stars to fit), so no star-shape judgment (tilt, backfocus, coma, tracking) is possible or relevant here.
Exposure-wise, the flat is good: median at 0.33 of full scale, black point at 0.17, 0% clipped pixels at both ends. It is a clean flat in terms of level, usable for calibration.
The only reservation concerns the illumination structure. An ideal flat should contain only the optical train's vignetting, hence a radially symmetric falloff. The measurements show the opposite: weak and poorly modeled radial falloff (center->edges +13%, R²=0.21) and above all a strong oriented-gradient component (plane at 337.5°, 37% amplitude, plane/radial dominance 2.82, low azimuthal anisotropy). This asymmetry betrays uneven illumination of the flat panel (or of the sky for a twilight flat), or even stray-light leakage. Applied as is, this flat risks introducing a residual gradient into the calibrated images instead of correcting one.
Priority actions
- Homogenize the flat source (EL panel/sky) and reshoot a series: aim for a symmetric distribution with no oriented gradient.
- Hunt for a possible light leak while taking the flats (cap, joints, misaligned flat screen).
- Keep the current dosing (median ~33%), which is ideal, and stack enough flats (20-30) to smooth the noise.