Astrophotography diagnosis of Eastern Veil Nebula - Foraxx: Clipped stars and over-denoising
ProcessedHa, OIII, SIIHa 129×300s · OIII 100×300s · SII 11×300s (total ~20.0 h)06 juil. 2026
The Doc examined this image of Eastern Veil Nebula - Foraxx (processed, Ha, OIII, SII, Ha 129×300s · OIII 100×300s · SII 11×300s (total ~20.0 h)). Estimated overall technical quality: 8/10. 2 defects found: Clipped stars (severity 2/5), Over-denoising (severity 2/5).
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Info
- Cible
- Eastern Veil Nebula - Foraxx
- Date
- 06 juil. 2026, 00:00
- Site
- Bortle 5 · banlieue résidentielle (saisi)
- Position
- 20h55m50s · +31°29'59"
Very favorable conditions for this target. Under Bortle 5 with a 27% moon, an emission nebula shot in Ha, OIII and SII is nearly immune to light pollution and moonglow: the narrow bands cut most of the urban continuum, and committing 20h here rather than waiting for a dark night was exactly the right call. The only filter really exposed to the sky background is OIII, more sensitive to gradient and moonlight, but with 100 subs of 300s the SNR absorbs it easily. Nothing to change in the planning strategy, you can repeat this scheme on every emission target.
Setup
- Type d'image
- Traitée
- Télescope
- Askar SQA70
- Caméra
- QHYCCD miniCAM8M
- Filtre
- Ha, OIII, SII
- Monture
- Te-Seek HM-17PE
- Exposition
- Ha 129×300s · OIII 100×300s · SII 11×300s (total ~20.0 h)
- Phase de lune
- 27%
- Notes
- Logiciels : DocStellar, Photopea, PHD2, PixInsight, BlurXTerminator, NoiseXTerminator, StarXTerminator, Nighttime Imaging 'N' Astronomy (N.I.N.A. / NINA)
- FOV
- 1.62°
The setup is well matched to the target. With 336mm at f/4.8 and a 1.62 degree field, the Eastern Veil sits comfortably in frame: the full crescent fits, with room on the right that lets the faint wisps breathe. The f/4.8 ratio is a real asset in narrowband, it compensates for the narrow bandpasses and explains why only 11 SII subs still hold up in the composite. Sampling gives measured FWHM of 2.6 to 3.3 px, right in the healthy range, neither over nor undersampled. On the histogram side, the background to black point margin of 0.95 is low but that is NORMAL narrowband behavior at 300s, not underexposure: do not try to lengthen the subs.
The diagnosis in detail
The acquisition is clean in the strict sense. The PSF panel gives a center elongation of 1.03 against a field floor of 1.02, a horizontal FWHM asymmetry of 0.006 and vertical of 0.007, and a corner to center FWHM ratio of 0.911, meaning the corners are actually slightly tighter than the center. So there is no field curvature, no tilt, no tracking drift: the SQA70 works within spec and the HM-17PE guided under PHD2 holds 300s without fault. The single 1.10 zone (BL) carries an ellipse fitted on a faint star at the edge of nebulosity, an isolated measurement that sets no trend.
The two reservations are therefore entirely in post processing, and they are consistent: the clipped bright star cores come from long subs not complemented by a short set, and the background smoothing comes from a NoiseXTerminator pass pushed hard on an already very clean background after 20h. Both can be fixed without going back under the sky, starting again from the master.
Comparison with the DSS confirms that everything visible in the dark areas, including the tenuous wisps along the right and bottom edges, corresponds to real nebulosity and not to artifacts.
Priority actions
- Acquire a short exposure set of 10 to 30s per filter to rebuild the saturated star cores
- Re-run NoiseXTerminator more gently in linear, with a luminance mask, to give the background back some micro texture
- Restart from the existing master for both fixes, no long reacquisition is needed
