Astrophotography diagnosis of SH2-131: Residual vignetting, clipped stars 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: 7/10. 3 defects found: Residual vignetting (severity 2/5), Clipped stars (severity 2/5), Background chroma noise (severity 2/5).
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Info
- Cible
- SH2-131
- Site
- Bortle 7 · périurbain pollué (saisi)
- Position
- 21h38m60s · +57°28'25"
Distinctly unfavorable conditions: a 95% Moon under a Bortle 7 sky is the worst combination for widefield work. The Antlia tri-band saves the shot by cutting most of the lunar and urban continuum, which is why the background remains usable, but the price is a still-loaded sky background and reduced contrast on the faint Ha extensions around the trunk. On an emission target like SH2-131 that filter choice is the right one. The 2h of integration, however, is not enough in this context: under a full Moon you typically need to double or triple the time to reach the SNR you would get on a dark night. If you can plan ahead, keep this target for a moonless slot and save full-Moon nights for pure Ha work.
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
- 2.33°
The setup is well matched to the target. The 80ED reduced to 0.85x gives a 2.33 degree field, which frames the whole of IC 1396 comfortably with the trunk nicely placed at lower right and margin all around. Sampling falls in the comfortable range for widefield, and the measured FWHM around 3 px confirms nothing is limited by the optics or the mount: the SAL-33 does its job with no detectable drift or oscillation. The Antlia tri-band is the right call under Bortle 7. One hardware caveat: the reducer increases edge-of-field illumination falloff, which makes flats critical, and they must be redone after any change to the imaging train.
The diagnosis in detail
Optically and mechanically, this acquisition is clean. The Moffat measurements give FWHM between 2.86 and 3.35 px across the nine zones, 1.04 elongation at center against a field floor of 1.03, a corner/center ratio of 1.041 and near-zero vertical asymmetry. In other words: no tilt, no backfocus error, no residual coma, and tracking free of drift or oscillation on 240s subs. That is an excellent foundation, the 80ED and the SAL-33 are both working within spec.
The three points raised share a common origin: the acquisition context. Two hours of exposure under a 95% Moon in Bortle 7 is a tight photon budget, and it shows directly in the sky background. The background-to-black-point margin of 2.22 confirms you are not comfortably sky-limited, and the color noise in the dark areas is the direct consequence. The clipping of bright star cores comes from the combination of long subs and an assertive stretch used to pull out the faint signal.
The 19% radial falloff is the only genuinely instrumental point. Its near-perfect rotational symmetry points to a flat imperfectly matched to the reduced imaging train, not to a light pollution gradient. It is the easiest to fix and will immediately improve background uniformity.
Priority actions
- Reshoot flats with the full imaging train assembled, nothing dismounted, to eliminate the 19% radial falloff
- Extend integration to at least 6h, ideally outside the full Moon period, to cut down background color noise
- Redo the stretch while protecting highlights, or add short subs to rebuild the star cores
- Apply noise reduction on chrominance only before the final stretch
