Astrophotography diagnosis of Western Veil: Light pollution gradient
ProcessedL-extreme47x300s
The Doc examined this image of Western Veil (processed, L-extreme, 47x300s). Estimated overall technical quality: 8/10. 1 defect found: Light pollution gradient (severity 3/5).
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Info
- Cible
- Western Veil
- Position
- 20h48m59s · +31°05'52"
Moon at 0%, so no lunar contribution: the measured gradient is purely terrestrial (light pollution or a low target). That is the best possible configuration for the Veil, and the L-eXtreme handles the rest against an urban background. With 47x300s (close to 4 hours) under a new moon you had every card in hand, and the resulting signal to noise confirms it, the fine filaments come through with little grain. The only improvement on the conditions side would be to concentrate the subs around culmination to reduce extinction and background brightness toward the bright horizon.
Setup
- Type d'image
- Traitée
- Télescope
- Askar fra400 with reducer 0.7x
- Caméra
- zwo asi533 mc pro
- Filtre
- L-extreme
- Monture
- am5n
- Exposition
- 47x300s
- Phase de lune
- 0%
- FOV
- 2.24°
A setup very well matched to the target. The FRA400 reduced to 0.7x gives 2.24 degrees of field, framing NGC 6960 and its filaments well without losing the object, even if the lacework spills slightly off the left edge. The square ASI533 avoids problematic corners and the PSF confirms it: 4 px of uniform FWHM, no sign of backfocus error or tilt, so the reducer spacing is correct. The L-eXtreme is the right choice on a supernova remnant rich in Ha and OIII. The 300s subs make sense in narrowband; the low background to black point margin (1.46) is normal here and does not indicate underexposure.
The diagnosis in detail
On the instrumental side there is nothing to fault. The Moffat measurements give an elongation of 1.02 in all nine zones, a PA dispersion of 29 degrees (the signature of round stars, not of a preferred direction) and a corner/center FWHM ratio of 1.111. Horizontal asymmetry of 0.097 stays below threshold: no tilt, no backfocus error, no coma. The AM5N tracking over 300s is clean, with no directional elongation at the center.
The only measured defect is the sky background. The plane model explains 93% of the variance with a 41% amplitude and dominates the radial model by a factor of 108: this is an oriented gradient at 312 degrees, not residual vignetting. Visually it shows as a warmer, brighter upper right corner that reduces local contrast on the faintest filaments and tints the background. With the Moon absent, the cause is on the ground.
The fix is fully recoverable in post: a careful gradient extraction on the linear image, with samples placed off the nebulosity (tricky here given how much of the frame the Veil covers), followed by background neutralization, should be enough. The processing is otherwise well controlled: stars are neither clipped nor ringed by dark halos, and the grain stays natural with no sign of over-denoising.
Priority actions
- Redo gradient extraction on the linear image with GraXpert RBF or DBE, placing samples only on genuine background
- Neutralize the sky background after extraction to remove the residual tint in the upper right corner
- Favor subs around culmination next time to reduce horizon background brightness
