Astrophotography diagnosis of SH2-131: Noise / underexposure, green cast and 1 other
ProcessedANTLIA tri band RGB Ultra 230x240s
The Doc examined this image of SH2-131 (processed, ANTLIA tri band RGB Ultra 2, 30x240s). Estimated overall technical quality: 6/10. 3 defects found: Noise / underexposure (severity 3/5), Green cast (severity 3/5), Background chroma noise (severity 2/5).
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Info
- Cible
- SH2-131
- Site
- Bortle 7 · périurbain pollué (saisi)
- Position
- 21h39m08s · +57°26'51"
Distinctly unfavourable conditions. A 95% moon combined with Bortle 7 pushes the sky background very high: even with the Antlia tri-band, which cuts most of the continuum, you lose a lot of contrast on a target as diffuse and faint as Sh2-131. The measured background/black point margin (2.18) confirms the useful signal stays thin despite 240s subs. The filter did its job, otherwise the image would be unusable, but it cannot compensate for a sky this bright. On this target, aim for nights with the moon below the horizon or under 30% illumination, and stack several sessions: that is the only lever that will really change the result.
Setup
- Type d'image
- Traitée
- Télescope
- skywatcher 80ED avec reducteur 0.85x
- Caméra
- ZWO ASI2600MC AIR
- Filtre
- ANTLIA tri band RGB Ultra 2
- Monture
- MLAStro SAL-33
- Exposition
- 30x240s
- Phase de lune
- 95%
- FOV
- 2.38°
Setup consistent with the target. The 80ED reduced at 0.85x gives a 2.38 degree field, which comfortably frames the whole Sh2-131 bubble with the Elephant Trunk at bottom right and margin at the edges: framing is not an issue. The measured FWHM around 2.8 px and the centre/corner consistency (ratio 1.034) show the reducer is correctly set in backfocus and the optical train is performing well, a point worth praising. The tri-band is the right choice on an HII region under moonlight. The weak link is not the hardware but the cumulative exposure time: 30x240s is short for this target in these conditions.
The diagnosis in detail
On the instrument side, nothing to fault. The Moffat measurements give elongation between 1.05 and 1.08 across the nine zones, against a field floor of 1.05, a corner/centre FWHM ratio of 1.034 and near-zero horizontal asymmetry. In other words the stars are round everywhere, corners included, and the PSF panel confirms it visually: the cyan ellipses hug symmetric disks. The automatic tilt pre-verdict rested on a vertical asymmetry of 0.106 driven mainly by the T zone (FWHM 3.3), with a measured confidence of only 44%; the panel shows no readable diagonal asymmetry, so I do not retain that defect. Tracking and backfocus are sound.
The real issue is signal to noise. Two hours under a near-full moon in Bortle 7 is too short for such a tenuous nebula: the background stays close to the black point, so the stretch had to be pushed, which lifted both luminance grain and chroma mottling in the dark areas at once.
On top of that sits a balance problem: the olive cast over the central area masks the Ha that should dominate in red-pink. That is fixable right now on the existing data, unlike the noise, which needs extra exposure time.
Priority actions
- Redo the colour calibration (SPCC or PCC) then a moderate SCNR to restore the red-pink Ha hue and neutralise the olive background
- Stack extra subs over several nights to reach 8 to 12 hours total on Sh2-131
- Reschedule the next sessions away from the moon, or with a low moon and illumination under 30%
- Reprocess separating luminance and chrominance to smooth colour noise heavily without crushing detail
- Leave the optical train alone: measured backfocus and field flatness are good
